workflows: Enable running the CI workflow manually

At least the Makefile workflow should be allowed to run on demand
for testing or in case the environment has changed.

Signed-off-by: Chuck Lever <[email protected]>

Author: chucklever

.github/workflows/makefile.yml

@@ -1,10 +1,6 @@
 name: Makefile CI
 
-on:
-  push:
-    branches: [ main ]
-  pull_request:
-    branches: [ main ]
+on: [ push, pull_request, workflow_dispatch ]
 
 jobs:
   build: