Add half baked permission checks to update command
It's hard to check the writability of the currently running executable, so for now just check the writability of the dir which seems to improve this situation a little? Especially as the issue reported in https://phabricator.wikimedia.org/T293963 is actually to do with the /usr/bin/.mw.new file, which I guess is the result of the download?
Perhaps in the future we should just stop using this update library all together and write our own?