Pin Swift 6.3.1 to Windows runners only

Swift 6.3.1's downloaded toolchain on macos-15 resolves against the
older Xcode 16.4 SDK's simd.swiftinterface and fails with thousands of
\"simd_floatNxM is not a member type\" errors. macOS and Linux were
green on 6.2.3 — only Windows needed the bump to satisfy the new MSVC's
Clang 20+ requirement.

Author: tomasf

.github/workflows/dev.yml

@@ -17,7 +17,7 @@ jobs:
     steps:
     - uses: SwiftyLab/setup-swift@latest
       with:
-        swift-version: "6.3.1"
+        swift-version: ${{ matrix.os == 'windows-latest' && '6.3.1' || '6.2.3' }}
     - uses: actions/checkout@v4
 
     - name: Install Linux dependencies

.github/workflows/main.yml

@@ -17,7 +17,7 @@ jobs:
     steps:
     - uses: SwiftyLab/setup-swift@latest
       with:
-        swift-version: "6.3.1"
+        swift-version: ${{ matrix.os == 'windows-latest' && '6.3.1' || '6.2.3' }}
     - uses: actions/checkout@v4
 
     - name: Install Linux dependencies