Actions: fix Actions for contributor PRs

- trigger on `pull_request`, not `push`.
- don't ignore branch names with a slash in them.  We don't use slashes,
  but external contributors often have branch names with slashes.
- we don't need to say anything about `coverity_scan`, because we're not
  opening PRs for it.

Also, fix the case of "runs-on: macOS-14"; it's documented as being
lower-case, so if they ever change this to be case-insensitive, the
previous version of this file would stop working.
(This point courtesy of Claude Sonnet 5.)

Author: gperciva

.github/workflows/compile.yml

@@ -1,11 +1,10 @@
 name: Compile & test
 
-# Run whenever we push to any branch without a "/", other than [coverity-scan].
+# Run whenever there's a new commit in a PR, or if we manually trigger one.
 on:
-  push:
-    branches:
-    - '*'
-    - '!coverity-scan'
+  pull_request:
+
+  workflow_dispatch:
 
 env:
   # Shared variables amongst all projects / platforms / compilers.
@@ -68,7 +67,7 @@ jobs:
       run: test -z "$(git status --porcelain=v1)"
   macOS:
     name: macOS
-    runs-on: macOS-14
+    runs-on: macos-14
     steps:
     - name: Checkout code
       uses: actions/checkout@v4