Allow forcing home-manager to use nix flake profiles instead of old nix with an option

[tnichols217]

Description

I added a new option to fix an odd bug where home-manager fails to activate if the .drvs are not present in the store. This happens because old nix needs them for some weird reason, and I'm not going to fix that, however nix flakes do not require them, so I created this option to allow forcing home-manager to use the new nix flake profiles instead of old nix.

Fixes #4928

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

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Maintainer CC

@rycee

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