Hide local timezone from VMs with specific tag

Set fake timezone for VMs with "anon-timezone" tag.
Fixes QubesOS/qubes-issues#8381

qubes/vm/qubesvm.py

@@ -2395,7 +2395,10 @@ def create_qdb_entries(self):
 
         tzname = qubes.utils.get_timezone()
         if tzname:
-            self.untrusted_qdb.write('/qubes-timezone', tzname)
+            if 'anon-timezone' in list(self.tags):
+                self.untrusted_qdb.write('/qubes-timezone', 'Etc/UTC')
+            else:
+                self.untrusted_qdb.write('/qubes-timezone', tzname)
 
         self.untrusted_qdb.write('/qubes-block-devices', '')
         self.untrusted_qdb.write('/qubes-usb-devices', '')