Handle case of MacOS updates to /etc/*rc more gracefully

[jonringer]

Introduce your stance
(I think the installer is part of this repo)

MacOS reserves the right to update /etc/bash.bashrc and /etc/zshrc. The problem is that these are destructive updates and causes the related files to revert the changes done by the nix installation.

Until mac supports /etc/zshrc.d/, we should probably have the installer handle the case that a /etc/*rc file was overwritten by mac.

Describe the issue
Explained above.

We should probably check if /etc/{bash.bash,zsh}rc contain nix, if they don't then use that as the new $file.backup-before-nix file, if they do contain references to nix, then skip attempting to modify it.

For the MacOS 12.3 case, the following would have been needed (copied from here):

# 1. Remove backup files
sudo rm /etc/{bashrc,zshrc}.backup-before-nix

# 2. Run installation script again
sh <(curl -L https://nixos.org/nix/install)

# Doing 2. before 1. makes the installation script fail.

[jonringer]

Related issues: #3616 #2280

[fricklerhandwerk]

@jonringer The installer is part of NixOS/nix.

[timfenney]

@fricklerhandwerk does this issue need to be moved?