Fix caching in GitHub Actions. (#519)

GitHub Actions cache is limited to 10 GiB and LRU, so with
21 matrix jobs, each storing multiple GiB per run, we were
constantly trashing cache and never restoring anything.

This change:
- prevents PRs from trashing cache,
- enables cache only for select jobs,
- stops caching things that we can download.

Signed-off-by: Piotr Sikora <code@piotrsikora.dev>

Author: PiotrSikora

.github/workflows/format.yml

@@ -90,10 +90,8 @@ jobs:
 
     - uses: bazel-contrib/setup-bazel@77a1d3d18379c7cb0a7e3b9fcaaa4d94f1029763 # 0.18.0
       with:
-        bazelisk-cache: true
         disk-cache: ${{ github.job }}
-        external-cache: false
-        repository-cache: true
+        cache-save: ${{ github.event_name != 'pull_request' }}
 
     - name: Format (bazel query)
       run: |
@@ -131,10 +129,8 @@ jobs:
 
     - uses: bazel-contrib/setup-bazel@77a1d3d18379c7cb0a7e3b9fcaaa4d94f1029763 # 0.18.0
       with:
-        bazelisk-cache: true
         disk-cache: ${{ github.job }}
-        external-cache: false
-        repository-cache: true
+        cache-save: ${{ github.event_name != 'pull_request' }}
 
     - name: Bazel build
       run: >

.github/workflows/test.yml

@@ -50,10 +50,8 @@ jobs:
 
     - uses: bazel-contrib/setup-bazel@77a1d3d18379c7cb0a7e3b9fcaaa4d94f1029763 # 0.18.0
       with:
-        bazelisk-cache: true
         disk-cache: ${{ github.job }}
-        external-cache: true
-        repository-cache: true
+        cache-save: ${{ github.event_name != 'pull_request' }}
 
     - name: Bazel build
       run: >
@@ -284,11 +282,10 @@ jobs:
       uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f # v3.12.0
 
     - uses: bazel-contrib/setup-bazel@77a1d3d18379c7cb0a7e3b9fcaaa4d94f1029763 # 0.18.0
+      if: ${{ matrix.cache }}
       with:
-        bazelisk-cache: true
         disk-cache: ${{ github.job }}-${{ steps.job-hash.outputs.hash }}
-        external-cache: true
-        repository-cache: true
+        cache-save: ${{ github.event_name != 'pull_request' }}
 
     - name: Download test data
       uses: actions/download-artifact@37930b1c2abaa49bbe596cd826c3c89aef350131 # v7.0.0