pull_request_target triggers the event=branch when it should only trigger on event=pr

[mithro]

@crazy-max -- I'm still getting a tag for the base branch (IE localhost:5000/run-drc-for-cell-gds-using-magic:main) - Have I configured things wrong or is pull_request_target getting misunderstood as being a branch/tag event as well as a pull request event? Output below;

  with:
    images: localhost:5000/run-drc-for-cell-gds-using-magic,gcr.io/skywater-pdk/actions/run-drc-for-cell-gds-using-magic
    tags: type=ref,event=tag
  type=ref,event=pr
  type=ref,event=branch
  type=sha
  type=sha,format=long
  
    github-token: ***
  env:
    ACTION_NAME: run-drc-for-cell-gds-using-magic
Context info
  eventName: pull_request_target
  sha: 1791415f66d16e92bdc523c59e68383db3b32d74
  ref: refs/heads/main
  workflow: Build Docker Image for Run DRC for cell GDS (using Magic) Action
  action: dockermetadata-action
  actor: mithro
  runNumber: 22
  runId: 875362048
Processing tags input
Processing flavor input
Docker image version
  pr-21
Docker tags
  localhost:5000/run-drc-for-cell-gds-using-magic:pr-21
  localhost:5000/run-drc-for-cell-gds-using-magic:main
  localhost:5000/run-drc-for-cell-gds-using-magic:sha-1791415
  localhost:5000/run-drc-for-cell-gds-using-magic:sha-1791415f66d16e92bdc523c59e68383db3b32d74
  gcr.io/skywater-pdk/actions/run-drc-for-cell-gds-using-magic:pr-21
  gcr.io/skywater-pdk/actions/run-drc-for-cell-gds-using-magic:main
  gcr.io/skywater-pdk/actions/run-drc-for-cell-gds-using-magic:sha-1791415
  gcr.io/skywater-pdk/actions/run-drc-for-cell-gds-using-magic:sha-1791415f66d16e92bdc523c59e68383db3b32d74

Originally posted by @mithro in #90 (comment)