utils: add support for testing self-hosted SPM

[compnerd]

-Test swiftpm will use the self-hosted build of SPM to run the test suite. By using the SWIFTCI_USE_LOCAL_DEPS=1 environment variable, we do reduce some of the cost of this by not recloning the repositories, but the entire dependency tree must be built a second time due to the switch in the build system.

[compnerd]

CC: @bnbarham @hyp @hjyamauchi

[compnerd]

@swift-ci please smoke test

[bnbarham]

Thanks 🙇‍♂️!

We can use this PR and cross-PR test in SPM to get it green. Either by making it default for now here, or by adding the -Test swiftpm to the existing job. (Or we can merge this, then have another PR that just enables it by default for cross-PR testing)

[compnerd]

I personally would prefer the latter - this functionality is useful for local development. #71349 will fail due to the -parse-as-library change being required to make progress.

[bnbarham]

Sure, sounds reasonable to me 👍 Thanks for putting that up already!

[compnerd]

Ah, found a small difference between my local usage and CI usage - will tweak this to make sure it works on both.

[compnerd]

@swift-ci please smoke test

[compnerd]

I am going to merge this for now as it improves the build.ps1 state to allow local development with this. swiftlang/swift-package-manager#7320 tracks the universal enablement while #71349 demonstrated that this does function on CI though the test suite crashes during execution.