util: check for setgid() as well in __normal_user()

Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27378>
(cherry picked from commit 501f78fdba)

src/util/u_debug.h

@@ -399,7 +399,7 @@ __normal_user(void)
 #if defined(_WIN32)
    return true;
 #else
-   return geteuid() == getuid();
+   return geteuid() == getuid() && getegid() == getgid();
 #endif
 }