Installer script isn't entirely idempotent

[colinta]

Platform

• Linux:
• macOS
• WSL

Additional information

I had installed nix long ago, but nix-build wasn't in my path so I assume it was removed or similar, so I ran the install script again.

I was surprised that the install script wasn't more idempotent – it wanted to recreate the _nixbld\d+ users, which caused errors. I was less upset about having to remove and rewrite the /etc/* files (fstab, etc), but there, too, it would be better if it noticed that these were already "as intended" by nix.

Checklist

• checked latest Nix manual (source)
• checked open installer issues and pull requests for possible duplicates

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