macOS Testing is broken with nightly toolchain

It appears the macOS run is broken, at least for runs that use a nightly toolchain. The "Install Swift toolchain via Swiftly" step fails with:

Run swiftly install main-snapshot
  swiftly install main-snapshot
  swiftly use main-snapshot
  shell: /bin/bash -e {0}
  env:
    DEVELOPER_DIR: /Applications/Xcode_26.4.app/Contents/Developer
    SWIFTLY_HOME_DIR: /Users/runner/.swiftly
    SWIFTLY_BIN_DIR: /Users/runner/.swiftly/bin
    SWIFTLY_TOOLCHAINS_DIR: /Users/runner/.swiftly/toolchains
Error: RunProgramError(exitCode: 1, program: "/usr/bin/xcrun", arguments: ["--show-sdk-path", "--sdk", "macosx"])
Swiftly is currently unlinked and will not manage the active toolchain. You can
run
the following command to link swiftly to the active toolchain:

    $ swiftly link


Fetching the latest main branch snapshot...
Error: Process completed with exit code 1.

https://github.com/swiftlang/swift-foundation/actions/runs/32396783384/job/96515349154

Start with the linked GitHub Actions job and inspect the macOS workflow's "Install Swift toolchain via Swiftly" step, including the xcrun SDK lookup and Swiftly link error. Verify the failure with the nightly toolchain and confirm that a macOS nightly run completes successfully after the workflow fix.