etc: allow overwriting files with unknown hashes

[Enzime]

For users encountering #149

[emilazy]

Is this still desirable now that we have known hashes for /etc/shells and /etc/nix/nix.conf, by the far the most common occurrences of this? I’m especially wary of a configuration‐based approach, as opposed to something more one‐time like an environment variable, as I suspect people will opt in and then forget about it. Overall I lean towards handling this cleanly upstream whenever we can and letting users manually mv things out of the way for the cases we can’t deal with ourselves.

[Enzime]

I agree that making this an option that users can set and forget in their configuration is not ideal, I'm happy to close this PR for now

The main reason I created this commit in the first place was for my darwin test VM where /etc/nix/nix.conf would not have a fixed content and I just wanted to ignore that, I'll have to think more potentially about how I'll implement it

[emilazy]

A potential simple solution would be an environment variable containing filenames or hashes that are okay to overwrite. You could also just add rm /etc/nix/nix.conf to the activation scripts if you wanted to do something really hacky :)