Code review comment for lp://staging/~rbalint/update-manager/do-release-upgrade-without-pkexec

I tested this on my system which is encountering a crash when trying a partial upgrade and now I have a different traceback:

Traceback (most recent call last):
  File "/usr/lib/ubuntu-release-upgrader/do-partial-upgrade", line 93, in <module>
    logdir = setup_logging(options, config)
  File "/usr/lib/python3/dist-packages/DistUpgrade/DistUpgradeMain.py", line 98, in setup_logging
    with open(fname, "a"):
PermissionError: [Errno 13] Permission denied: '/var/log/dist-upgrade/main.log.partial'