Change diff reference for admin UI check

Previously, this was pointing to the file from the base commit
of the PR. While this works fine for PRs that target the "main"
branch, when a PR targets any other branch, we'll want to reference
the branch's name rather than a specific commit SHA.

At least that's what I think is going on.

.github/workflows/check-admin-ui-api.yml

@@ -33,7 +33,7 @@ jobs:
           npx graphql-inspector diff \
             --rule .github/workflows/check-admin-ui-api/rule.js \
             --onComplete .github/workflows/check-admin-ui-api/onComplete.js \
-            git:${{ github.event.pull_request.base.sha }}:frontend/src/schema.graphql \
+            git:${{ github.event.pull_request.base.ref }}:frontend/src/schema.graphql \
             git:${{ github.event.pull_request.head.sha }}:frontend/src/schema.graphql \
             | tee >( \
               grep -v ^::set-output \