How to get a reference to the PR's target branch for go-apidiff

[abbasadel]

Hi,

We are using go-apidiff to check golang code backward compatibility part of the CI.
For go-apidiff to work properly, it needs to have $GITHUB_BASE_REF of the PR's target branch which is not available during bros merge and produces the following error:

 ##[group]Run joelanford/go-apidiff@main
 with:
   base-ref: origin/${GITHUB_BASE_REF}
   version: latest
   compare-imports: false
   print-compatible: true
   repo-path: .
   skip-cache: false
 env:
   DOCKER_PLATFORMS: linux/amd64,linux/arm64
   ARTIFACTS_USR: ***
   ARTIFACTS_PSW: ***
 + /home/runner/go/bin/go-apidiff origin/ --compare-imports=false --print-compatible=true --repo-path=.
 failed to lookup git commit hashes: could not get hash for "origin/": Revision invalid : must not end with "/"
 ##[error]Process completed with exit code 2.
 Post job cleanup.

Is there a way to set $GITHUB_BASE_REF or have a way to get a reference to the PR's target branch during bros merge?
More info: camunda/camunda#12838

Thanks