libghostty: enable cross-compiling macOS from Linux/Windows

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.

.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: