CI: use x64-native cross tools to build x86 code

This is also used by the Swift CI:
swiftlang/swift-corelibs-libdispatch#606 (comment)

See https://learn.microsoft.com/en-us/cpp/build/building-on-the-command-line

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        arch: [x86, x64]
+        arch: [x64_x86, x64]
 
     steps:
       - uses: actions/checkout@v3