fix: add actions:write permission for GHA cache

Without actions:write, --cache-to type=gha silently fails with 403,
resulting in no cache being stored and cold builds every time.

Author: wo-o

.github/workflows/push-ghcr.yml

@@ -15,6 +15,7 @@ jobs:
   build_and_push:
     runs-on: ubuntu-latest
     permissions:
+      actions: write
       contents: read
       packages: write