.github: less verbose on: specifier

The `branch:` specifier was necessary before, to prevent double-testing
master and trunk.  Now that we no longer have trunk, we can slim the
expressions.

See also: https://help.github.com/en/actions/reference/workflow-syntax-for-github-actions#on

.github/workflows/macos.yml

@@ -1,11 +1,5 @@
 name: macOS
-on:
+on: [push, pull_request]
-  push:
-    branches:
-      - '*'
-  pull_request:
-    branches:
-      - '*'
 jobs:
   make:
     runs-on: macos-latest

.github/workflows/mingw.yml

@@ -1,11 +1,5 @@
 name: MinGW
-on:
+on: [push, pull_request]
-  push:
-    branches:
-      - '*'
-  pull_request:
-    branches:
-      - '*'
 
 # Notes:
 # Action ENV TEMP and TMP are short 8.3 paths, but the long path differs.

.github/workflows/mjit.yml

@@ -1,11 +1,5 @@
 name: MJIT
-on:
+on: [push, pull_request]
-  push:
-    branches:
-      - '*'
-  pull_request:
-    branches:
-      - '*'
 jobs:
   make:
     strategy:

.github/workflows/ubuntu.yml

@@ -1,11 +1,5 @@
 name: Ubuntu
-on:
+on: [push, pull_request]
-  push:
-    branches:
-      - '*'
-  pull_request:
-    branches:
-      - '*'
 jobs:
   make:
     strategy:

.github/workflows/windows.yml

@@ -1,11 +1,5 @@
 name: Windows
-on:
+on: [push, pull_request]
-  push:
-    branches:
-      - '*'
-  pull_request:
-    branches:
-      - '*'
 jobs:
   make:
     strategy: