Extract common build steps to composite actions (#242)

* Extract common build steps to composite actions

This allows us to repeat ourselves less across the different jobs.

I also tested using a “reusable workflow” to factor out the common
steps. However, this starts a separate job without a shared
filesystem, which in turn requires us to upload/download artifacts
when we want to use them in several jobs. The artifacts are downloaded
one-by-one and this adds delays and extra steps to all jobs.

* Move Rust cache setup to its own build step

This made it easy to consistently setup the caching of our nested
projects via the “workspacs” config key.

.github/workflows/install-mdbook/action.yml

@@ -0,0 +1,18 @@
+name: Install mdbook and dependencies
+
+description: Install the mdbook with the dependencies we need.
+
+runs:
+  using: composite
+  steps:
+    - name: Install mdbook
+      run: cargo install mdbook --version 0.4.25
+      shell: bash
+
+    - name: Install mdbook-svgbob
+      run: cargo install mdbook-svgbob --version 0.2.1
+      shell: bash
+
+    - name: Install i18n-helpers
+      run: cargo install --path i18n-helpers --locked
+      shell: bash