build: Don't re-run Mega Linter on main branch (only PRs)

Signed-off-by: Michael Vorburger <mike@vorburger.ch>

.github/workflows/mega-linter.yaml

@@ -6,22 +6,22 @@
 # see https://megalinter.io/latest/mega-linter-vs-super-linter/
 name: MegaLinter
 
-# Trigger mega-linter at every push.
-# Action will also be visible from Pull Requests to main
 on:
-  push:
+  # DO NOT run on push: but only on Pull Requests
+  # As this repository is configured to enforce "never push directly to main"
+  # (it would say: [remote rejected] main -> main (protected branch hook declined)),
+  # there doesn't seem to be a point to re-run Mega Linter after PRs are merged.
+  # One reason to change this could be to configure this to let it raise PRs to
+  # apply fixes to main, but the overall workflow for that isn't very clear
+  # to me yet, because PRs would still be red, so... ?
 
   pull_request:
     branches:
       - main
-      - master
 
 # Comment env block if you do not want to apply fixes
 env:
   # Apply linter fixes configuration
-  #
-  # When active, APPLY_FIXES must also be defined as environment variable
-  # (in github/workflows/mega-linter.yml or other CI tool)
   APPLY_FIXES: all
 
   # Decide which event triggers application of fixes in a commit or a PR