Non-privileged actions performed with GID=135 (mock)

Hopefully, we do not open a can of worms.., but using GID=135 for
non-privileged operations seems like the most expected Mock behavior.
This way, we automatically maintain the ownership-bridge between the
on-host user and the in-chroot user (both present in the 'mock' group).

Mock started to map the unprivGid to the default GID of the on-host user
in the commit 5cce73a, which is very
old and rather seems like a mistake rather than intention.

Fixes: #1257

mock/py/mockbuild/uid.py

@@ -22,21 +22,19 @@
 def setup_uid_manager():
     mockgid = grp.getgrnam('mock').gr_gid
     unprivUid = os.getuid()
-    unprivGid = os.getgid()
+    unprivGid = mockgid
 
     # sudo
     if os.environ.get("SUDO_UID") is not None:
         unprivUid = int(os.environ['SUDO_UID'])
         os.setgroups((mockgid,))
-        unprivGid = int(os.environ['SUDO_GID'])
 
     # consolehelper
     if os.environ.get("USERHELPER_UID") is not None:
         unprivUid = int(os.environ['USERHELPER_UID'])
         unprivName = pwd.getpwuid(unprivUid).pw_name
         secondary_groups = [g.gr_gid for g in grp.getgrall() if unprivName in g.gr_mem]
         os.setgroups([mockgid] + secondary_groups)
-        unprivGid = pwd.getpwuid(unprivUid)[3]
 
     uidManager = UidManager(unprivUid, unprivGid)
     return uidManager