[Buildsystem] Ping master cache after saving cache

This should improve cache retention on large builds.

(cherry picked from commit 8e4e1451ec2d03761446d50ed3b14338d15a8b0e)

.github/actions/godot-cache-save/action.yml

@@ -20,3 +20,14 @@ runs:
       with:
         path: ${{ inputs.scons-cache }}
         key: ${{ inputs.cache-name }}|${{ github.ref_name }}|${{ github.sha }}
+
+    # This is done after saving the new cache so that the base cache remains more relevant, to reduce
+    # the risk of the cache being purged when multiple large PRs are pushed/merged at the same time.
+    - name: Ping default cache
+      uses: actions/cache/restore@v6
+      if: github.ref_name != github.event.repository.default_branch
+      with:
+        path: ${{ inputs.scons-cache }}
+        key: ${{ inputs.cache-name }}|${{ github.event.repository.default_branch }}|${{ github.sha }}
+        restore-keys: ${{ inputs.cache-name }}|${{ github.event.repository.default_branch }}
+        lookup-only: true