Bundle CI workflows on main via release.yml

Change lint, test, and variants workflows to trigger on pull_request
instead of push, so they only run independently on PRs. On push to
main, release.yml calls them as reusable workflows.

Author: jensens

.github/workflows/lint.yml

@@ -1,7 +1,7 @@
 name: Lint
 
 on:
-  push:
+  pull_request:
   workflow_call:
   workflow_dispatch:
 

.github/workflows/test.yml

@@ -1,7 +1,7 @@
 name: Tests
 
 on:
-  push:
+  pull_request:
   workflow_call:
   workflow_dispatch:
 

.github/workflows/variants.yml

@@ -1,7 +1,7 @@
 name: Python/VENV/Installer Variants
 
 on:
-  push:
+  pull_request:
   workflow_call:
   workflow_dispatch: