Issue Description:

Hello dev:

when using [swift-windows-toolchain 2020 12 20 snapshot](https://dev.azure.com/compnerd/swift-build/_build/results?buildId=44103&view=artifacts&pathAsName=false&type=publishedArtifacts)

to build a demo project

[compnerd/swift-win32](https://github.com/compnerd/swift-win32)

by `SPM`, but `SPM` build fail like below:

```

c:\Users\User\Downloads\swift-win32>swift build TSCUtility/Triple.swift:152: Fatal error: could not determine host triple: malformed error: unable to invoke subcommand: C:\Library\Developer\Toolchains\unknown-Asserts-development.xctoolchain\usr\bin\swift-build.exe ()

```

And

@compnerd said:

> I happened to come across this on GitHub actions once. I think that this is a bug in SPM, and is transient. Re-running the command seemed to be fine on GitHub at least. I think that reporting the issue on bugs.swift.org might be reasonable.

> [More detail](https://github.com/compnerd/swift-win32/issues/327#issuecomment-751395160)

The solution maybe metioned on :
google/swift-jupyter#103 (comment)