nu: fix build on Apple Silicon

[cho-m]

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[cho-m]

I don't think this works as intended on macOS since it checks if path exists (https://github.com/programming-nu/nu/blob/v2.3.0/tools/nuke#L280)

It first checks clang is a path which fails. Then tries #{sdk}clang which fails. Finally, it sets gcc.

However, since gcc is an alias for clang on macOS, there is no actual impact, so is probably okay.

[BrewTestBot]

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