Use simpler build matrix, split by OS

Rather than a single matrix that covers all OSes, split them up
so that enumerating different ones is a little easier.

This has three benefits:

1. `matrix.exclude` is easier to write in this format
2. Tools like `act` work with simple string matrix dimensions
   (see new comment at top of maven.yml)
3. conditional execution on file name pattens are easier to implement
   (see subsequent PR on this topic)

Part of #2714

Author: jonahgraham

.github/workflows/maven.yml

@@ -1,5 +1,13 @@
 # This is the main maven workflow for SWT.
 
+# You can run this locally to test changes with the act command https://nektosact.com/
+# For example, to run all GTK4 tests:
+#   act -j build-linux --matrix java:21 --artifact-server-path $PWD/.artifacts
+# You may need to download runners on first run, act will ask but if you
+# want the big runner there is no progress as it downloads, so you can pull it with
+# docker directly:
+#   docker pull catthehacker/ubuntu:full-latest
+
 name: SWT Matrix Build
 concurrency: 
     group: ${{ github.workflow }}-${{ github.ref }}
@@ -25,19 +33,48 @@ jobs:
       with:
         name: Event File
         path: ${{ github.event_path }}
-  build:
+
+  build-linux:
+    name: Build (Linux)
+    strategy:
+      fail-fast: false
+      matrix:
+        java: ['21']
+    uses: ./.github/workflows/build.yml
+    with:
+      runner: ubuntu-latest
+      java: ${{ matrix.java }}
+      native: gtk.linux.x86_64
+      performance: ${{ contains(github.event.pull_request.labels.*.name, 'performance') }}
+
+  build-windows:
+    name: Build (Windows)
+    strategy:
+      fail-fast: false
+      matrix:
+        java: ['21']
+    uses: ./.github/workflows/build.yml
+    with:
+      runner: windows-latest
+      java: ${{ matrix.java }}
+      native: win32.win32.x86_64
+
+  build-macos:
+    name: Build (macOS)
     strategy:
       fail-fast: false
       matrix:
-        java: [ '21' ]
-        config: 
-          - { name: Linux, os: ubuntu-latest, native: gtk.linux.x86_64 }
-          - { name: Windows, os: windows-latest, native: win32.win32.x86_64 }
-          - { name: MacOS x86, os: macos-15-intel, native: cocoa.macosx.x86_64 }
-          - { name: MacOS ARM, os: macos-latest, native: cocoa.macosx.aarch64 }
+        java: ['21']
+        native: [cocoa.macosx.x86_64, cocoa.macosx.aarch64]
+        runner: [macos-15-intel, macos-latest]
+        exclude:
+          - runner: macos-latest
+            native: cocoa.macosx.x86_64
+          - runner: macos-15-intel
+            native: cocoa.macosx.aarch64
     uses: ./.github/workflows/build.yml
     with:
-      runner: ${{ matrix.config.os }}
+      runner: ${{ matrix.runner }}
       java: ${{ matrix.java }}
-      native: ${{ matrix.config.native }}
+      native: ${{ matrix.native }}
       performance: ${{ contains(github.event.pull_request.labels.*.name, 'performance') }}