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 <cherryyz@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

misc/cgo/test/cgo_linux_test.go

@@ -5,6 +5,7 @@
 package cgotest
 
 import (
+	"os"
 	"runtime"
 	"testing"
 )
@@ -13,13 +14,19 @@ func TestSetgid(t *testing.T) {
 	if runtime.GOOS == "android" {
 		t.Skip("unsupported on Android")
 	}
+	if _, err := os.Stat("/etc/alpine-release"); err == nil {
+		t.Skip("setgid is broken with musl libc - go.dev/issue/39857")
+	}
 	testSetgid(t)
 }
 
 func TestSetgidStress(t *testing.T) {
 	if runtime.GOOS == "android" {
 		t.Skip("unsupported on Android")
 	}
+	if _, err := os.Stat("/etc/alpine-release"); err == nil {
+		t.Skip("setgid is broken with musl libc - go.dev/issue/39857")
+	}
 	testSetgidStress(t)
 }