GHA CI: only store compile cache on stable branches

Given the amount of PR and compile matrix, the CI cache size limit is
easy to hit. So for now on, only store compile cache for stable branches
to avoid cache thrashing.

.github/workflows/ci_macos.yaml

@@ -46,6 +46,7 @@ jobs:
       - name: Setup ccache
         uses: Chocobo1/setup-ccache-action@v1
         with:
+          store_cache: ${{ startsWith(github.ref, 'refs/heads/') }}
           update_packager_index: false
 
       - name: Install boost

.github/workflows/ci_ubuntu.yaml

@@ -40,6 +40,7 @@ jobs:
       - name: Setup ccache
         uses: Chocobo1/setup-ccache-action@v1
         with:
+          store_cache: ${{ startsWith(github.ref, 'refs/heads/') }}
           update_packager_index: false
           ccache_options: |
             max_size=2G