ci: cache builds with sccache

.github/workflows/build-docs.yaml

@@ -1,4 +1,6 @@
 # Sets permissions of the GITHUB_TOKEN to allow deployment to GitHub Pages
+name: "Build documentation"
+
 permissions:
   contents: read
   pages: write
@@ -9,13 +11,28 @@ on:
     branches:
       - main
 
-jobs:
+env:
+  SCCACHE_VERSION: v0.7.7
 
+jobs:
   build:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - uses: dtolnay/rust-toolchain@nightly
+      - name: Setup rust toolchain
+        uses: dtolnay/rust-toolchain@nightly
+      - name: Obtain sccache
+        run: |
+          curl -fsSL https://github.com/mozilla/sccache/releases/download/${SCCACHE_VERSION}/sccache-${SCCACHE_VERSION}-x86_64-unknown-linux-musl.tar.gz \
+            | tar xz sccache-${SCCACHE_VERSION}-x86_64-unknown-linux-musl/sccache --strip=1
+      - name: Configure sccache
+        uses: actions/github-script@v6
+        with:
+          script: |
+            core.exportVariable('RUSTC_WRAPPER', './sccache')
+            core.exportVariable('SCCACHE_GHA_ENABLED', 'on')
+            core.exportVariable('ACTIONS_CACHE_URL', process.env.ACTIONS_CACHE_URL || '');
+            core.exportVariable('ACTIONS_RUNTIME_TOKEN', process.env.ACTIONS_RUNTIME_TOKEN || '');
       - name: Install mdBook and dependencies
         run: |
           cargo install mdbook mdbook-admonish