ci(build): save the compiler cache from main builds

No job saved its ccache, so every build restored a day-old cache and
recompiled about half the tree (971/2038 hits). Main push builds now
save it, and they no longer cancel each other, so one finishes and
refreshes it. Pull request builds still only restore, so PR code never
writes the shared cache.

.github/workflows/build.yml

@@ -18,7 +18,7 @@ permissions:
 
 concurrency:
   group: build-${{ github.event_name }}-${{ github.event.pull_request.number || inputs.pr || github.ref }}
-  cancel-in-progress: true
+  cancel-in-progress: ${{ github.event_name != 'push' }}
 
 env:
   CONFIGURE: >-
@@ -200,7 +200,7 @@ jobs:
       - uses: hendrikmuhs/ccache-action@v1.2.24
         with:
           key: linux
-          save: false
+          save: ${{ github.event_name == 'push' && github.ref == 'refs/heads/main' }}
           evict-old-files: job
       - name: Install lavapipe for the GPU execution tests
         run: sudo apt-get update && sudo apt-get install -y --no-install-recommends libvulkan1 mesa-vulkan-drivers
@@ -264,7 +264,7 @@ jobs:
       - uses: hendrikmuhs/ccache-action@v1.2.24
         with:
           key: windows
-          save: false
+          save: ${{ github.event_name == 'push' && github.ref == 'refs/heads/main' }}
           evict-old-files: job
       - shell: cmd
         run: cmake -S . -B build %CONFIGURE%