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)

.pick_status.json

@@ -204,7 +204,7 @@
         "description": "util: check for setgid() as well in __normal_user()",
         "nominated": true,
         "nomination_type": 0,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": null,
         "notes": null

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
 }