Fixup: use effective git ref to test-update jobs

... especially for cross edition update jobs as they are cron triggered
and get no CUSTOM_GIT_REF handed over explicitly. A re-run of a previously
failed job has started a new build again.

Change-Id: Ibad15950f865032185a668cde88cdaac402ac3c3

buildscripts/scripts/test-update.groovy

@@ -92,7 +92,7 @@ def main() {
                         EDITION: edition,
                         VERSION: version,
                         CROSS_EDITION_TARGET: cross_edition_target,
-                        CUSTOM_GIT_REF: CUSTOM_GIT_REF,
+                        CUSTOM_GIT_REF: effective_git_ref,
                         FAKE_WINDOWS_ARTIFACTS: params.FAKE_WINDOWS_ARTIFACTS,
                     ],
                     build_params_no_check: [