skip checkup / skip init fail to compile SkipPlugin.swift under Xcode 27: "unknown argument: '-target-arch-variant'"

Summary

skip checkup and skip init (with a brand-new, unmodified project) both fail before ever reaching user code or skip-ui — the build-tool plugin bundled with this package, SkipPlugin.swift, fails to compile.

This started after updating to Xcode 27.0 / macOS 27.0 (Golden Gate). The identical Skip/Swift/Swiftly toolchain versions worked cleanly on Xcode 26.6 six days earlier (skip checkup passed all twelve checks, skip init + skip android build succeeded). No project code, dependencies, or Skip/Swift/Swiftly versions changed between the two runs — only the OS/Xcode update.

This is unrelated to #521 in skip-ui (a separate skip-ui regression we reported on Sep 9). That bug never gets reached here, because this failure happens earlier, in this package's own plugin.

Environment

Skip	1.9.8
macOS	27.0 (Golden Gate), build 26A428
Xcode	27.0, build 27A266a
Swift	6.3.3 (swiftly 1.1.3)
Swift Android SDK	6.3.3
Gradle	9.7.1 · OpenJDK 26.0.2.1 · adb 1.0.41 · Android SDK 37.0.1

Six days earlier, on the same machine, with identical Skip/Swift/Swiftly/Gradle/SDK versions but Xcode 26.6 / macOS 26.6.2: skip checkup passed all twelve checks, and skip init + skip android build both ran (surfacing the unrelated skip-ui #521 instead).

Steps to reproduce

skip checkup

or, from an empty directory:

skip init --transpiled-app --appid=com.example.spike --no-fastlane --no-git-repo spike Spike

Both fail identically. skip checkup's own internal "Build hello-skip" step hits the same error.

Actual result

[✗] Build hello-skip (4.68s)
[✗] error: emit-module command failed with exit code 1 (use -v to see invocation)
<unknown>:0: error: unknown argument: '-target-arch-variant'

(repeated 20 times), followed by a cascade in this package's own plugin:

.build/checkouts/skip/Plugins/SkipPlugin/SkipPlugin.swift:43:87: error: cannot find type 'Command' in scope
.build/checkouts/skip/Plugins/SkipPlugin/SkipPlugin.swift:69:108: error: cannot find type 'Command' in scope
.build/checkouts/skip/Plugins/SkipPlugin/SkipPlugin.swift:76:108: error: cannot find type 'Command' in scope
.build/checkouts/skip/Plugins/SkipPlugin/SkipPlugin.swift:6:14: error: type 'SkipPlugin' cannot conform to protocol 'BuildToolPlugin' because it has requirements that cannot be satisfied
.build/checkouts/skip/Plugins/SkipPlugin/SkipPlugin.swift:379:26: error: cannot find type 'LocalizedError' in scope
.build/checkouts/skip/Plugins/SkipPlugin/SkipPlugin.swift:108:23: error: thrown expression type 'SkipPluginError' does not conform to 'Error'
.build/checkouts/skip/Plugins/SkipPlugin/SkipPlugin.swift:158:25: error: cannot find 'URL' in scope
.build/checkouts/skip/Plugins/SkipPlugin/SkipPlugin.swift:256:38: error: cannot find 'URL' in scope

then, for skip init specifically:

error: build planning stopped due to build-tool plugin failures
[✗] Skip 1.9.8 init failed with 2 errors

Command, URL, and LocalizedError are ordinary Foundation/PackagePlugin types — this reads as a cascade from the first rejected compiler invocation, not as separate missing imports.

Expected result

skip checkup and skip init succeed under Xcode 27, as they did under Xcode 26.6.

Notes

• -target-arch-variant looks like a driver flag mismatch — plausibly the host-side compiler invocation used to build SwiftPM build-tool plugins under Xcode 27 emits a flag that the swiftly-installed Swift 6.3.3 frontend doesn't recognize, even though swift --version and skip checkup's own version check both report 6.3.3 as matching.
• We did not change swiftly, the Android SDK, or any Skip/skip-ui version between the two test runs — only the OS.
• Happy to provide -v verbose invocation output, or any other diagnostic, on request.

Start by reproducing the failure with skip checkup or the provided skip init command, then inspect Plugins/SkipPlugin/SkipPlugin.swift and the verbose compiler invocation. Compare the Xcode 27 invocation with the known-working Xcode 26.6 environment. Done means both commands compile the plugin and succeed under Xcode 27.