Fix test-acl failure on AIX due to read only LOGNAME.

On AIX /etc/profile sets LOGNAME read only. This causes test-acl to
fail when it comes to set LOGNAME in do_push().

Work around this by using env to set LOGNAME and run the command.

tests/test-acl

@@ -14,7 +14,11 b' do_push()'
 	cat acl.config
 	echo '"""'
     fi
-    LOGNAME=$user hg --cwd a --debug push ../b
+    # On AIX /etc/profile sets LOGNAME read-only. So
+    #  LOGNAME=$user hg --cws a --debug push ../b
+    # fails with "This variable is read only."
+    # Use env to work around this.
+    env LOGNAME=$user hg --cwd a --debug push ../b
     hg --cwd b rollback
     hg --cwd b --quiet tip
     echo