preserve quotes for set input fields

[crazy-max]

follow-up docker/setup-buildx-action#175

There are cases where quotes need to be used for multi values fields like name for image type output:

set: |
  *.output=type=image,"name=moby/buildkit:v0.11.0,moby/buildkit:latest",push=true

Currently it would fail with: https://github.com/crazy-max/buildkit/actions/runs/3275927287/jobs/5391622084#step:7:122

  /usr/bin/docker buildx bake --set *.cache-from=type=gha,scope=cross --set *.cache-from=type=gha,scope=image --set *.cache-to=type=gha,scope=image --set *.output=type=image,name=***/buildkit:v0.11.0,***/buildkit:latest,buildinfo-attrs=true,push=true --set *.tags= --metadata-file /tmp/docker-build-push-1iSyRZ/metadata-file image-all --print
  ERROR: invalid value ***/buildkit:latest

Signed-off-by: CrazyMax crazy-max@users.noreply.github.com