Use the right branch for fetching dependencies

scripts/fetchdep.sh

@@ -17,10 +17,8 @@ clone() {
     fi
 }
 
-# Try the PR author's branch in case it exists on the deps as well.
-clone $TRAVIS_PULL_REQUEST_BRANCH
 # Try the target branch of the push or PR.
-clone $TRAVIS_BRANCH
+clone $BUILDKITE_BRANCH
 # Try the current branch from Jenkins.
 clone `"echo $GIT_BRANCH" | sed -e 's/^origin\///'`
 # Use the default branch as the last resort.