misc/cgo/test: disable setgid tests with musl
misc/cgo/test: disable setgid tests with musl

We don't have a good musl detection mechanism, so we detect Alpine (the
most common user of musl) instead.

For #39857.
For #19938.

Change-Id: I2fa39248682aed75884476374fe2212be4427347
Reviewed-on: https://go-review.googlesource.com/c/go/+/425001
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Auto-Submit: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go