Cache CI test artefacts (#2)

.github/workflows/test.yml

@@ -1,3 +1,4 @@
+name: Test
 on: push
 
 jobs:
@@ -9,5 +10,43 @@ jobs:
         with:
           version-type: strict
           version-file: .tool-versions
+      - name: Cache deps
+        id: cache-deps
+        uses: actions/cache@v3
+        env:
+          cache-name: cache-elixir-deps
+        with:
+          path: deps
+          key: ${{ runner.os }}-mix-${{ env.cache-name }}-${{ hashFiles('**/mix.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-mix-${{ env.cache-name }}-
+
+      # Step: Define how to cache the `_build` directory. After the first run,
+      # this speeds up tests runs a lot. This includes not re-compiling our
+      # project's downloaded deps every run.
+      - name: Cache compiled build
+        id: cache-build
+        uses: actions/cache@v3
+        env:
+          cache-name: cache-compiled-build
+        with:
+          path: _build
+          key: ${{ runner.os }}-mix-${{ env.cache-name }}-${{ hashFiles('**/mix.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-mix-${{ env.cache-name }}-
+            ${{ runner.os }}-mix-
+
+      # Step: Conditionally bust the cache when job is re-run.
+      # Sometimes, we may have issues with incremental builds that are fixed by
+      # doing a full recompile. In order to not waste dev time on such trivial
+      # issues (while also reaping the time savings of incremental builds for
+      # *most* day-to-day development), force a full recompile only on builds
+      # that are retried.
+      - name: Clean to rule out incremental build as a source of flakiness
+        if: github.run_attempt != '1'
+        run: |
+          mix deps.clean --all
+          mix clean
+        shell: sh
       - run: mix deps.get
       - run: mix test