workflows/treadmill-ci-test: run on pushes to main branch

.github/workflows/treadmill-ci-test.yml

@@ -10,7 +10,7 @@ env:
 on:
   push:
     branches:
-      - master
+      - main
   # Pull requests from forks will not have access to the required GitHub API
   # secrets below, even if they are using an appropriate deployment environment
   # and the workflow runs have been approved according to this environment's