ci: fix buildx bake push-by-digest error by not overriding output name

[Virakal]

Fix buildx bake failure caused by overriding the output 'name' without tags.

docker-bake.hcl already generates per-target tags. Overriding the output 'name' to the repository (without tags) caused buildx to attempt pushing tagged refs by digest which Docker Hub rejects. Removing the name override lets buildx use the tags defined by bake and avoids the "can't push tagged ref ... by digest" error.

Files changed:

• .github/workflows/multi-platform-build.yml

How to verify:

• Run the workflow in GitHub Actions (manual dispatch) or run 'bin/bake.js --print' locally to review output.
• Optionally run a non-pushing or load-local build with 'docker buildx bake php --set php.platforms=linux/amd64 --load' and verify local images.