Allow cross-PR testing for GitHub actions

[ahoppen]

See https://github.com/swiftlang/github-workflows/blob/main/README.md#cross-pr-testing

Eg. needed for #9557

[ahoppen]

@swift-ci Please test

[bkhouri]

This is a great initiative. However, unless I missed something, when I browsed the https://github.com/swiftlang/github-workflows repository code base, setting enable_cross_pr_testing to true will only clone repositories, but the swift build does not actually use them.

Do we need to so something else to ensure the build command uses the "linked PRs"?

[ahoppen]

setting enable_cross_pr_testing to true will only clone repositories, but the swift build does not actually use them.

It does by setting a package mirror to point to the local checkout, which has the referenced PR merged: https://github.com/swiftlang/github-workflows/blob/61db6917df4ec2ddeeb493757e2d1effd46af855/.github/workflows/scripts/cross-pr-checkout.swift#L225-L234

[ahoppen]

@swift-ci Please test macOS