fix: fall back to getent for users that are not findable in /etc/passwd

[tonyandrewmeyer]

The go User.Lookup method on Unix-like systems without ego just looks in the /etc/password file (whereas cgo does a getpwnam check via libc). On some systems, like the Canonical laptops, the user that is trying to run Concierge may not be in /etc/passwd and be loaded from a directory service instead.

This PR adds a fallback system where we will run the getent binary. That comes from the libc-bin package in Ubuntu, so is very likely to be available (and the worst case is that it's not and we'll fail just as we did before).

Fixes #182

[tonyandrewmeyer]

FYI, you can test this with the --dry-run argument on a Canonical laptop. With the latest release (or main) you should get an error before Concierge manages to do anything. With this branch (sudo go run ./main.py prepare -p dev --dry-run) you should see the normal dry-run output. You won't see the call to getent because that happens so early (there's a comment that explains this); I looked into using that instead but it made it quite complex for almost no benefit.

[dimaqq]

Fly-by: the change looks good to me.