Make fetchdep check out matching branch name (#29601)

* Make fetchdep check out matching branch name

on a push to a branch.

* Remove buildkite support entirely

scripts/fetchdep.sh

@@ -45,10 +45,7 @@ getPRInfo() {
 
 # Some CIs don't give us enough info, so we just get the PR number and ask the
 # GH API for more info - "fork:branch". Some give us this directly.
-if [ -n "$BUILDKITE_BRANCH" ]; then
-    # BuildKite
-    head=$BUILDKITE_BRANCH
-elif [ -n "$PR_NUMBER" ]; then
+if [ -n "$PR_NUMBER" ]; then
     # GitHub
     getPRInfo $PR_NUMBER
 elif [ -n "$REVIEW_ID" ]; then
@@ -79,11 +76,14 @@ if [[ "$GITHUB_EVENT_NAME" == "merge_group" ]]; then
     clone $deforg $defrepo ${withoutPrefix%%/pr-*}
 fi
 
-# Try the target branch of the push or PR.
-if [ -n "$GITHUB_BASE_REF" ]; then
-    clone $deforg $defrepo $GITHUB_BASE_REF
-elif [ -n "$BUILDKITE_PULL_REQUEST_BASE_BRANCH" ]; then
-    clone $deforg $defrepo $BUILDKITE_PULL_REQUEST_BASE_BRANCH
+# Try the target branch of the push or PR, or the branch that was pushed to
+# (ie. the 'master' branch should use matching 'master' dependencies)
+base_or_branch=$GITHUB_BASE_REF
+if [[ "$GITHUB_EVENT_NAME" == "push" ]]; then
+    base_or_branch=${GITHUB_REF}
+fi
+if [ -n "$base_or_branch" ]; then
+    clone $deforg $defrepo $base_or_branch
 fi
 
 # Try HEAD which is the branch name in Netlify (not BRANCH which is pull/xxxx/head for PR builds)