libghostty: enable cross-compiling macOS from Linux/Windows (#12417)

This allows libghostty-vt to be cross-compiled for macOS from non-macOS
platforms. I've updated pkg/apple-sdk to fallback to Zig's embedded
macOS headers if the macOS SDK is not found. Additionally,
CombineArchivesStep has been updated to use Linux tooling on Linux. CI
updated to test this.

.github/workflows/test.yml

@@ -569,14 +569,15 @@ jobs:
   build-libghostty-vt:
     strategy:
       matrix:
-        target:
-          [
+        target: [
             aarch64-macos,
             x86_64-macos,
             aarch64-linux,
             x86_64-linux,
             x86_64-linux-musl,
-            x86_64-windows,
+            x86_64-windows-gnu,
+            # doesn't work yet, we need a way to find msvc libc/c++ headers
+            # x86_64-windows-msvc
             wasm32-freestanding,
           ]
     runs-on: namespace-profile-ghostty-sm
@@ -607,8 +608,7 @@ jobs:
       - name: Build
         run: |
           nix develop -c zig build -Demit-lib-vt \
-            -Dtarget=${{ matrix.target }} \
-            -Dsimd=false
+            -Dtarget=${{ matrix.target }}
 
   # lib-vt requires macOS runner for macOS/iOS builds because it requires the `apple_sdk` path
   build-libghostty-vt-macos: