fix(test): fix current_branch for backwards compat test (#18565)

ci/workflows/main-cron.yml

@@ -712,6 +712,8 @@ steps:
           run: source-test-env
           config: ci/docker-compose.yml
           mount-buildkite-agent: true
+          environment:
+            - BUILDKITE_BRANCH
       - ./ci/plugins/upload-failure-logs
     matrix:
       setup:

ci/workflows/pull-request.yml

@@ -711,6 +711,8 @@ steps:
           run: source-test-env
           config: ci/docker-compose.yml
           mount-buildkite-agent: true
+          environment:
+            - BUILDKITE_BRANCH
       - ./ci/plugins/upload-failure-logs
     matrix:
       setup:

e2e_test/backwards-compat-tests/scripts/utils.sh

@@ -138,7 +138,10 @@ get_old_version() {
 
   # We handle the edge case where the current branch is the one being released.
   # If so, we need to prune it from the list.
-  local current_branch=$(git branch --show-current | tr -d 'v')
+  # We cannot simply use 'git branch --show-current', because buildkite checks out with the commit,
+  # rather than branch. So the current state is detached.
+  # Instead we rely on BUILDKITE_BRANCH, provided by buildkite.
+  local current_branch=$(echo "$BUILDKITE_BRANCH" | tr -d 'v')
   echo "--- CURRENT BRANCH: $current_branch"
 
   echo "--- PRUNED VERSIONS"
@@ -147,7 +150,7 @@ get_old_version() {
 
   # Then we take the Nth latest version.
   # We set $OLD_VERSION to this.
-  OLD_VERSION=$(echo -e "$sorted_versions" | tail -n $VERSION_OFFSET | head -1)
+  OLD_VERSION=$(echo -e "$pruned_versions" | tail -n $VERSION_OFFSET | head -1)
 }
 
 get_new_version() {