further restrict polaris workflow runs

only run on pushes to main and manual dispatches via web interface

.github/workflows/end_to_end_regression_polaris.yml

@@ -7,12 +7,7 @@ on:
     paths:
       - darshan-runtime/**
       - .github/workflows/end_to_end_regression_polaris.yml
-  pull_request:
-    branches:
-      - main
-    paths:
-      - darshan-runtime/**
-      - .github/workflows/end_to_end_regression_polaris.yml
+  workflow_dispatch:
 
 jobs:
   trigger_alcf_polaris_regression_ci: