tests: disable apidiff check (#316)

internal/check_api_change.sh

@@ -21,11 +21,11 @@
 # This script expects:
 # a) to be run at the root of the repository
 # b) HEAD is pointing to a commit that merges between the pull request and the
-#    upstream branch (GITHUB_HEAD_REF).
+#    upstream branch (GITHUB_BASE_REF).
 
 set -euo pipefail
 
-UPSTREAM_BRANCH="${GITHUB_HEAD_REF:-master}"
+UPSTREAM_BRANCH="${GITHUB_BASE_REF:-master}"
 echo "Checking for incompatible API changes relative to ${UPSTREAM_BRANCH}..."
 
 MASTER_CLONE_DIR="$(mktemp -d)"