os/user: fix build with "osusergo" build tag

Fixes #24841
Updates #24845

Change-Id: I4a5c05f4cbf9692bd6cab48baf3cc51fa43fe5a9
Reviewed-on: https://go-review.googlesource.com/106837Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/user/cgo_unix_test.go

@@ -3,7 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build darwin dragonfly freebsd !android,linux netbsd openbsd solaris
-// +build cgo
+// +build cgo,!osusergo
 
 package user
 

src/os/user/getgrouplist_unix.go

@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build dragonfly freebsd !android,linux netbsd openbsd
+// +build cgo,!osusergo
 
 package user