Fix build warnings in the workflow for iOS

[kkebo]

IMHO, it would be better to use $(xcrun -f swift) ("find" mode) instead of xcrun swift ("run" mode).

xcrun swift is fine with packages that do not contain executableTarget, but it causes the following errors with packages that contain executableTarget:

clang: warning: using sysroot for 'MacOSX' but targeting 'iPhone'
[-Wincompatible-sysroot]

To prevent this, we can use $(xcrun -f swift) instead of xcrun swift.

Related discussion:
https://forums.swift.org/t/getting-incompatible-sysroot-warning-when-compiling-for-ios-with-sdk-and-target-set/80584

You can easily reproduce this issue by the following steps:

mkdir foo && cd foo
swift package init --type executable --name Foo
xcrun swift build --sdk "$(xcrun --sdk iphoneos --show-sdk-path)" --triple arm64-apple-ios

[shahmishal]

@grynspan would like to get you feedback here

[grynspan]

Fine with me!

[jakepetroules]

To be clear, building for iOS isn't really supported with swift build today, so this is kind of a best-effort situation at the moment and a bit clunky to get it to build at all. Once we have the new Swift Build backend in place, it will become formally supported though (and I already got it working in main). The correct invocation will then become:

swift build --build-tests --triple arm64-apple-ios

without needing to mess with xcrun or manually pass down the sysroot.

I'm fine with having the workflow up in the meantime though, as it's still useful for the cases where it does work. The invocation you have now looks good to me.