Multi-User Darwin Installer: No side effects before old installation check

[Ericson2314]

I think there are a few instances of the installer doing things before deciding whether to abort, which makes retrying a real pain as one has delete files over and over again.

Instances (to be edited):

• Catalina volume and /etc/synthetic.conf /nix creation. This one macos has made especially irksome, as one can easily create /nix, but must reboot to get rid of it.

[abathur]

I'm marking #4289 as closing this, but it's not a 100% overlap with how you formulated this, so I'll edit it back out if you object.

• I did move the volume setup process into the multi-user installer, so it does now take place after the "is-nix-installed?" check.
• But, a previous PR (but commits unreleased) has already converted the nix-exists error into a "this may not work" sort of warning in master, so there's less of a meaningful hard wall.
• My PR adds a ~curing process that should make the installer resilient to the existence of synthetic.conf, /nix, and the volume.
• generic (non-Darwin) install cruft is out-of-scope, so it'll still be possible to hit hard stops on things like the /etc/*.backup-before-nix files and such (until we make some sort of phase-2 effort to extend the foundation work in this PR to all of the artifacts)