ci: add GNU-ABI Windows library build job

The existing `build-libghostty-vt-windows` job builds libghostty-vt
with the MSVC ABI. The Win32 apprt (discussion #2563) will target
the GNU ABI, so add a parallel job that exercises the GNU-ABI path
to catch bitrot.

The job runs `zig build -Dtarget=native-native-gnu -Dapp-runtime=none`
which produces ghostty-vt.dll and ghostty-internal.dll without
requiring a platform-specific apprt.

.github/workflows/test.yml

@@ -96,6 +96,7 @@ jobs:
       - build-libghostty-vt-android
       - build-libghostty-vt-macos
       - build-libghostty-vt-windows
+      - build-libghostty-windows-gnu
       - build-linux
       - build-linux-libghostty
       - build-nix
@@ -713,6 +714,20 @@ jobs:
       - name: Build libghostty-vt
         run: zig build -Demit-lib-vt
 
+  build-libghostty-windows-gnu:
+    runs-on: namespace-profile-ghostty-windows
+    timeout-minutes: 45
+    needs: test
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      - name: Setup Zig
+        uses: mlugg/setup-zig@d1434d08867e3ee9daa34448df10607b98908d29 # v2.2.1
+
+      - name: Build libghostty (GNU ABI)
+        run: zig build -Dtarget=native-native-gnu -Dapp-runtime=none
+
   build-linux:
     strategy:
       fail-fast: false