ci: listen for workflow_dispatch

By adding this, you can use gh workflow command or the UI to trigger
runs.

Signed-off-by: Bill Roberts <bill.c.roberts@gmail.com>

Author: williamcroberts

.github/workflows/main.yml

@@ -1,6 +1,6 @@
 name: Linux Build Status
 on:
-  [push, pull_request]
+  [push, pull_request, workflow_dispatch ]
 jobs:
   build-test:
     runs-on: ubuntu-latest