ci: link swiftly so SwiftBuild can locate its resource bundles

Replaces the --build-system native workaround with `swiftly link`.

When swiftly is left unlinked after `swiftly init --skip-install`,
SwiftBuild's resource_bundle_accessor cannot resolve bundles such as
SwiftBuild_SWBUniversalPlatform from the toolchain's usr/share/pm
directory, crashing the build before any compilation.

Linking the active toolchain exposes it via swiftly's managed PATH so
SwiftBuild finds its resources, letting us build with the default
(swiftbuild) system.

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

Author: pblazej

.github/workflows/swift-snapshot.yaml

@@ -30,10 +30,11 @@ jobs:
         run: brew install swiftly && swiftly init --quiet-shell-followup --skip-install -y
 
       - name: Get Swift snapshot
-        run: swiftly install -y ${{ env.SWIFT_SNAPSHOT }}
+        # `swiftly link` ensures SwiftBuild can locate its resource bundles
+        # (e.g. SwiftBuild_SWBUniversalPlatform) via the toolchain path.
+        run: |
+          swiftly install -y ${{ env.SWIFT_SNAPSHOT }}
+          swiftly link
 
       - name: Build
-        # --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 +${{ env.SWIFT_SNAPSHOT }} --build-system native
+        run: swiftly run swift build +${{ env.SWIFT_SNAPSHOT }}