Include toolchain search paths in the build description signature

[cybercussion]

Fixes #1798.

Problem

The Metal Toolchain component is a cryptex mounted at /var/run/com.apple.security.cryptexd/mnt/com.apple.MobileAsset.MetalToolchain-v<version>.<random>, and the random suffix changes on every mount (e.g. after a reboot). SwiftPM passes that mount point to the build service as EXTERNAL_TOOLCHAINS_DIR (SwiftBuildSystem.swift), and tool paths resolved from it (TOOLCHAIN_DIR, the metal compiler) are baked into the build description.

BuildDescriptionSignatureComponents records nothing about where toolchains were found. After a remount the cached description is reused, and the build fails:

error: unable to spawn process '/var/run/com.apple.security.cryptexd/mnt/com.apple.MobileAsset.MetalToolchain-v27.1.266.1.zFisgc/Metal.xctoolchain/usr/bin/metal' (No such file or directory)

(The toolchain was now mounted at …gTiXky, and it works.)

Fix

This follows the pattern of #1792 (sdkInputsSignature):

• Core.toolchainSearchPathsSignature: the resolved toolchain search paths, including EXTERNAL_TOOLCHAINS_DIR entries.
• It is included in BuildDescriptionSignatureComponents, so a moved toolchain forces a fresh build description.

Unchanged search paths give an identical signature, so a normal incremental build still reuses its build description.

Testing

• New CoreTests.toolchainSearchPathsSignatureTracksExternalToolchainMount: the same Metal toolchain under two different mount suffixes gives different signatures, and the same mount gives equal ones.
• swift test --filter "SWBTaskExecutionTests|SWBCoreTests": 598 tests pass. The 2 known issues were already marked as known (MultiPlatformDependencyResolverTests).
• Found in real use: a package depending on SwiftTerm (which ships Shaders.metal), on Xcode 27.0 (27A266a), macOS 26.6.2, MetalToolchain v27.1.266.1, swift build failing the morning after a reboot.

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