support merge queue runs, use SHAs for default base ref

Signed-off-by: Joe Lanford <joe.lanford@gmail.com>

action.yml

@@ -7,7 +7,7 @@ inputs:
   base-ref:
     description: 'Base reference for API compatibility comparison (default: base branch)'
     required: false
-    default: "origin/${GITHUB_BASE_REF}"
+    default: "${{ github.event.pull_request.base.sha || github.event.merge_queue.base_sha }}"
   version:
     description: 'Version of go-apidiff to use'
     required: false