CI: Add restore cache and run on targeted pipeline related jobs only

FYI: The targeted 'pipeline' related jobs are:
- Mobile App
- Lint
- Unit Tests
- Android Tests (Commented Out)

PS.1: The 'detekt' and 'checkstyle' jobs aren't targeted because they
only take about 30+ seconds of network request to download all the
dependencies needed for those specific jobs, which is about the same
time it takes for the dependency cache to actually be restored (20+
seconds). Thus, this optimization is not really worth it for these
specific jobs. For all those other jobs targeted, it take more than a
minute of network requests to download all the dependencies needed, and
as such worth the diff is worth it.
PS.2: Although the 'Android Tests' are commented-out, I chose to include
cache restoration into those as well, just in case they will be
reactivated at some point in the future.

.buildkite/commands/restore-cache.sh

@@ -0,0 +1,6 @@
+#!/bin/bash
+
+set -e
+
+echo "--- 💾 Restore Cache"
+restore_gradle_dependency_cache || true

.buildkite/pipeline.yml

@@ -52,13 +52,17 @@ steps:
           - "**/build/reports/detekt/detekt.html"
 
       - label: "🕵️ Lint WordPress"
-        command: ".buildkite/commands/lint.sh" "wordpress"
+        command: |
+          .buildkite/commands/restore-cache.sh
+          ".buildkite/commands/lint.sh" "wordpress"
         plugins: [$CI_TOOLKIT]
         artifact_paths:
           - "**/build/reports/lint-results*.*"
 
       - label: "🕵️ Lint Jetpack"
-        command: ".buildkite/commands/lint.sh" "jetpack"
+        command: |
+          .buildkite/commands/restore-cache.sh
+          ".buildkite/commands/lint.sh" "jetpack"
         plugins: [$CI_TOOLKIT]
         artifact_paths:
           - "**/build/reports/lint-results*.*"
@@ -75,7 +79,9 @@ steps:
   - group: "🔬 Unit Tests"
     steps:
       - label: "🔬 Unit Test WordPress"
-        command: ".buildkite/commands/run-unit-tests.sh" "wordpress"
+        command: |
+          .buildkite/commands/restore-cache.sh
+          ".buildkite/commands/run-unit-tests.sh" "wordpress"
         plugins:
           - $CI_TOOLKIT
           - $TEST_COLLECTOR :
@@ -85,7 +91,9 @@ steps:
           - "**/build/test-results/merged-test-results.xml"
 
       - label: "🔬 Unit Test Processors"
-        command: ".buildkite/commands/run-unit-tests.sh" "processors"
+        command: |
+          .buildkite/commands/restore-cache.sh
+          ".buildkite/commands/run-unit-tests.sh" "processors"
         plugins:
           - $CI_TOOLKIT
           - $TEST_COLLECTOR :
@@ -95,7 +103,9 @@ steps:
           - "**/build/test-results/merged-test-results.xml"
 
       - label: "🔬 Unit Test Image Editor"
-        command: ".buildkite/commands/run-unit-tests.sh" "image-editor"
+        command: |
+          .buildkite/commands/restore-cache.sh
+          ".buildkite/commands/run-unit-tests.sh" "image-editor"
         plugins:
           - $CI_TOOLKIT
           - $TEST_COLLECTOR :
@@ -105,7 +115,9 @@ steps:
           - "**/build/test-results/merged-test-results.xml"
 
       - label: "🔬 Unit Test FluxC"
-        command: ".buildkite/commands/run-unit-tests.sh" "fluxc"
+        command: |
+          .buildkite/commands/restore-cache.sh
+          ".buildkite/commands/run-unit-tests.sh" "fluxc"
         plugins:
           - $CI_TOOLKIT
           - $TEST_COLLECTOR:
@@ -115,7 +127,9 @@ steps:
           - "**/build/test-results/merged-test-results.xml"
 
       - label: "🔬 Unit Test Login"
-        command: ".buildkite/commands/run-unit-tests.sh" "login"
+        command: |
+          .buildkite/commands/restore-cache.sh
+          ".buildkite/commands/run-unit-tests.sh" "login"
         plugins:
           - $CI_TOOLKIT
           - $TEST_COLLECTOR:
@@ -130,7 +144,9 @@ steps:
   # - group: "🔬 Instrumented tests"
   #   steps:
   #     - label: ":wordpress: 🔬 Instrumented tests"
-  #       command: ".buildkite/commands/run-instrumented-tests.sh" "wordpress"
+  #       command: |
+  #         .buildkite/commands/restore-cache.sh
+  #         "".buildkite/commands/run-instrumented-tests.sh" "wordpress"
   #       plugins:
   #         - $CI_TOOLKIT
   #         - $TEST_COLLECTOR :
@@ -140,7 +156,9 @@ steps:
   #         - "**/build/instrumented-tests/**/*"
 
   #     - label: ":jetpack: 🔬 Instrumented tests"
-  #       command: ".buildkite/commands/run-instrumented-tests.sh" "jetpack"
+  #       command: |
+  #         .buildkite/commands/restore-cache.sh
+  #         ".buildkite/commands/run-instrumented-tests.sh" "jetpack"
   #       plugins:
   #         - $CI_TOOLKIT
   #         - $TEST_COLLECTOR :
@@ -155,11 +173,15 @@ steps:
   - group: "📲 Prototype Builds"
     steps:
       - label: ":wordpress: :android: Prototype Build"
-        command: ".buildkite/commands/prototype-build.sh" "wordpress"
+        command: |
+          .buildkite/commands/restore-cache.sh
+          ".buildkite/commands/prototype-build.sh" "wordpress"
         if: build.pull_request.id != null
         plugins: [$CI_TOOLKIT]
 
       - label: ":jetpack: :android: Prototype Build"
-        command: ".buildkite/commands/prototype-build.sh" "jetpack"
+        command: |
+          .buildkite/commands/restore-cache.sh
+          ".buildkite/commands/prototype-build.sh" "jetpack"
         if: build.pull_request.id != null
         plugins: [$CI_TOOLKIT]