ci(build): Add packages:write permission to snapshot workflow (#3227)

## Summary

- Add missing `packages: write` permission to the snapshot workflow so
that the called `build.yml` nested jobs (`platform-specific-docker`,
`multiarch-docker`) can push Docker images.

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Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>

Author: runningcode

.github/workflows/snapshot.yml

@@ -6,6 +6,7 @@ on:
 
 permissions:
   contents: write
+  packages: write
 
 jobs:
   prepare: