fix: Trigger A/B-Test runs by $GITHUB_REF_NAME

Currently, our runs get triggered on `/ref/heads/main`. For consistency
with other pipelines, normalize that to just `main`.

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

.github/workflows/trigger_ab_tests.yml

@@ -16,7 +16,7 @@ jobs:
                -H 'Authorization: Bearer ${{ secrets.BUILDKITE_TOKEN }}' \
                -d '{
                     "commit": "HEAD",
-                    "branch": "${{ github.event.ref }}",
+                    "branch": "$GITHUB_REF_NAME",
                     "env": {
                       "REVISION_A": "${{ github.event.before }}",
                       "REVISION_B": "${{ github.event.after }}"