ci: skip cache update on PRs. (proxy-wasm#244)

Cache updates on PRs result in early eviction of primary cache,
due to the limited cache quota on GitHub Actions.

While there, remove fallback to restore cache key based only on
the Wasm runtime name, since it prevents pruning of stale data.

Signed-off-by: Piotr Sikora <[email protected]>

Author: PiotrSikora

.github/workflows/cpp.yml

@@ -66,7 +66,7 @@ jobs:
     - uses: actions/checkout@v2
 
     - name: Bazel cache
-      uses: actions/cache@v2
+      uses: PiotrSikora/cache@v2.1.7-with-skip-cache
       with:
         path: |
           ~/.cache/bazel
@@ -88,7 +88,12 @@ jobs:
         if-no-files-found: error
         retention-days: 3
 
+    - name: Skip Bazel cache update
+      if: ${{ github.ref != 'refs/heads/master' }}
+      run: echo "CACHE_SKIP_SAVE=true" >> $GITHUB_ENV
+
     - name: Cleanup Bazel cache
+      if: ${{ github.ref == 'refs/heads/master' }}
       run: |
         export OUTPUT=$(bazel info output_base)
         # Distfiles for Rust toolchains (350 MiB).
@@ -179,15 +184,14 @@ jobs:
 
     - name: Bazel cache
       if: ${{ matrix.runtime != 'wasmtime' || startsWith(matrix.run_under, 'docker') }}
-      uses: actions/cache@v2
+      uses: PiotrSikora/cache@v2.1.7-with-skip-cache
       with:
         path: |
           ~/.cache/bazel
           /private/var/tmp/_bazel_runner/
         key: ${{ matrix.arch }}-${{ matrix.os }}-${{ matrix.runtime }}-${{ steps.cache-key.outputs.uniq }}-${{ hashFiles('WORKSPACE', '.bazelrc', '.bazelversion', 'bazel/dependencies.bzl', 'bazel/repositories.bzl') }}
         restore-keys: |
           ${{ matrix.arch }}-${{ matrix.os }}-${{ matrix.runtime }}-${{ steps.cache-key.outputs.uniq }}-
-          ${{ matrix.arch }}-${{ matrix.os }}-${{ matrix.runtime }}
 
     - name: Download test data
       uses: actions/download-artifact@v2
@@ -224,8 +228,12 @@ jobs:
         --per_file_copt=src/signature_util.cc,test/signature_util_test.cc@-DPROXY_WASM_VERIFY_WITH_ED25519_PUBKEY=\"$(xxd -p -c 256 test/test_data/signature_key1.pub | cut -b9-)\"
         //test:signature_util_test
 
+    - name: Skip Bazel cache update
+      if: ${{ github.ref != 'refs/heads/master' && (matrix.runtime != 'wasmtime' || startsWith(matrix.run_under, 'docker')) }}
+      run: echo "CACHE_SKIP_SAVE=true" >> $GITHUB_ENV
+
     - name: Cleanup Bazel cache
-      if: ${{ matrix.runtime != 'wasmtime' || startsWith(matrix.run_under, 'docker') }}
+      if: ${{ github.ref == 'refs/heads/master' && (matrix.runtime != 'wasmtime' || startsWith(matrix.run_under, 'docker')) }}
       run: |
         export OUTPUT=$(bazel info output_base)
         # BoringSSL's test data (90 MiB).