Fix branch of reproducer pipeline

.gitlab-ci.yml

@@ -43,6 +43,6 @@ test:
 test_reproducible:
   stage: check_reproducibility
   script:
-    - "curl -X POST -F token=${RELEASE_CHECK_TOKEN} -F ref=main -F variables[APP]='briar-mailbox' -F variables[RELEASE_TAG]=${CI_COMMIT_REF_NAME} https://code.briarproject.org/api/v4/projects/61/trigger/pipeline"
+    - "curl -X POST -F token=${RELEASE_CHECK_TOKEN} -F ref=master -F variables[APP]='briar-mailbox' -F variables[RELEASE_TAG]=${CI_COMMIT_REF_NAME} https://code.briarproject.org/api/v4/projects/61/trigger/pipeline"
   only:
     - tags