Checkout *elastic*'s master branch

Needed because I hacked around branch selection because we don't have
a `feature/sql` branch in the public elasticsearch repo.

Original commit: elastic/x-pack-elasticsearch@3fc1084588

dev-tools/ci

@@ -116,6 +116,7 @@ if [ -z ${USE_EXISTING_ES:+x} ]; then
     }
 
     # pick_clone_target NOCOMMIT just use master for testing our feature branch. Do not merge this.....
+    GH_USER="elastic"
     BRANCH="master"
 
     echo " -> checking out '$BRANCH' branch from $GH_USER/elasticsearch..."