revert hardcoded branch filter for testing on branch

Author: mosche

.buildkite/scripts/run-bc-upgrade-tests.sh

@@ -16,7 +16,7 @@ set -euo pipefail
 # So if we have one (or more) BC, we will always prefer to use that. Otherwise we will use the latest snapshot.
 MANIFEST_URL="$(curl -s https://artifacts.elastic.co/releases/TfEVhiaBGqR64ie0g0r0uUwNAbEQMu1Z/future-releases/stack.json |
 jq ".releases[] |
-select(.branch == \"main\") |
+select(.branch == \"$BUILDKITE_BRANCH\") |
 select(.active_release == true) |
 ((.snapshots | to_entries | sort_by(.value.completed_at)) +
 (.build_candidates | to_entries | sort_by(.value.completed_at))) |