Use SwiftSyntax prebuilt action in CI

[kateinoigakukun]

Wires swiftwasm/setup-swift-syntax-prebuilts@v1 into the Linux SwiftSyntax-heavy CI jobs and passes the action's SwiftPM flags into the relevant swift/Makefile/script invocations.

This deliberately does not cache SwiftPM .build/ directories. SwiftPM build products are not guaranteed to be cross-machine portable, so the PR only relies on the action's toolchain/platform-scoped SwiftSyntax prebuilt artifacts.

Action-side prerequisites are now in place:

• swiftwasm/setup-swift-syntax-prebuilts@v1 points at the merged Node 24 action runtime.
• Published release assets now exist for JavaScriptKit's current development snapshot:
  • prebuilt-603.0.2-swift-DEVELOPMENT-SNAPSHOT-2026-05-27-a with Ubuntu 22.04 and Ubuntu 24.04 assets.
  • prebuilt-600.0.1-swift-DEVELOPMENT-SNAPSHOT-2026-05-27-a with the Ubuntu 22.04 asset used by check-bridgejs-generated.
• The action repo also has a follow-up branch (yt/support-swiftpm-triple-build-dir) that fixes build-artifact discovery for SwiftPM's newer triple-qualified build output layout; that was needed to publish the May 2026 assets.

Current timing evidence:

Job	Latest main	PR evidence	Notes
Ubuntu 24.04 development	15m00s	14m33s in run 27335043949	Prebuilts restored from GitHub Release, but make unittest still compiles SwiftSyntax as a normal package dependency.
Ubuntu 24.04 Swift 6.3 branch	12m29s	13m38s in run 27335043949	Same limitation; small regression likely from action overhead plus run variance.
Ubuntu 22.04 development	20m57s	20m48s in run 27335043949	Essentially flat; PackageToJS and BridgeJS integration tests dominate.
check-bridgejs-generated	3m31s	3m36s in run 27335043949	Flat; action restore adds a few seconds.
build-examples	56m56s	57m45s in run 27335043949	Flat/slower; example builds dominate and .build/ is intentionally not cached.
swift:6.3 BridgeJS matrix	3m12s	3m14s in run 27338274284	The action is disabled for this job because the Docker image reports compiler tag swift-6.3.2-RELEASE, which has no matching release asset and previously caused a slow local prebuild fallback.

Why the speedup is small:

• The action does restore assets (Restore source: github-release) for the May 2026 Linux jobs.
• SwiftPM still compiles SwiftSyntax libraries when JavaScriptKit/BridgeJS depend on them directly (SwiftSyntax, SwiftParser, SwiftSyntaxBuilder, SwiftSyntaxMacros, test-support targets, etc.). The prebuilt action does not replace those normal library dependency builds in the observed CI logs.
• The longest steps are integration/example paths: PackageToJS's package build finished in ~17s, but its ExampleTests.basic() took ~7.5m; build-examples remains ~58m.
• We are intentionally not caching .build/, so those package/example builds are cold each run.

Expected CI signal:

• Covered setup steps should report Restore source: github-release or github-cache.
• They should not fall back to local-build for the covered SwiftSyntax/toolchain/platform combinations.
• There is no actions/cache usage for SwiftPM .build/ directories in this PR.