Set up typegraphs cache dir as a symlink on CI

Our goal is to create a persistent cache of svg files between builds.

When a CI agent is first created, the cache will populate the svg
files in the build agent's home directory. Subsequent builds should
be faster. Refs #393

.buildkite/pipeline.yaml

@@ -4,7 +4,7 @@ steps:
   - label: "Build site"
     key: build
     commands:
-      - mkdir -p /home/builder/cache
+      - ./.buildkite/setup-typegraphs-cache.sh
       - zef install . --deps-only
       - ./bin_files/build-site --without-completion --no-status
       - ./bin_files/build-site --no-status EBook

.buildkite/setup-typegraphs-cache.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+#
+set -e
+
+relpath="Website/plugins/typegraph/typegraphs"
+abspath="/home/builder/cache/typegraphs"
+
+if [ ! -s $relpath ]; then
+  rm -rf $relpath
+fi
+
+ln -s $abspath $relpath
+