Automatically backup filename collisions during home-manager installation

[nikhil-057]

Description

I have a git repo with home.nix and dotfiles, which I use to bootstrap all my non-nixos VMs.

Home-manager installation usually fails because most distros come with some user-level dotfiles by default (such as ~/.profile), and my home-manager config attempts to create these dotfiles, resulting in collisions.

The home-manager cli already provides a -b EXT option. However, nix-shell "<home-manager>" -A install runs a hard-coded shellHook that does not make use of this option.

I feel that the simplest solution is to allow users to customize the shellHook by turning it into a function arg, taking advantage of the callPackage mechanism to override the shellHook as the user sees fit. I've already opened a PR implementing this, but happy for the maintainers to implement any solution that solves this issue.