ci: use native build system for swift snapshot workflow

Recent main-snapshot toolchains crash at build startup with
"unable to find bundle named SwiftBuild_SWBUniversalPlatform"
(SwiftPM's SwiftBuild integration cannot locate its resource bundles
on the CI image). This prevents the job from ever reaching Swift
compilation.

Pass --build-system native to fall back to the classic SwiftPM build
system, which is sufficient for the workflow's goal of validating the
package against upcoming Swift.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pblazej

.github/workflows/swift-snapshot.yaml

@@ -33,4 +33,7 @@ jobs:
         run: swiftly install -y ${{ env.SWIFT_SNAPSHOT }}
 
       - name: Build
-        run: swiftly run swift build +${{ env.SWIFT_SNAPSHOT }}
+        # --build-system native avoids a SwiftBuild resource-bundle crash
+        # ("unable to find bundle named SwiftBuild_SWBUniversalPlatform")
+        # seen in recent main-snapshot toolchains on the CI image.
+        run: swiftly run swift build +${{ env.SWIFT_SNAPSHOT }} --build-system native