home-manager: support username with special chars

[DDoSolitary]

Description

The home manager script fails when $USER contains special characters.

For example, my work PC is managed by company's LDAP and username is <COMPANY>\<user>). When running home-manager switch I get the following error:

error: flake 'path:/home/<COMPANY>/<user>/.config/home-manager' does not provide attribute 'packages.x86_64-linux.homeConfigurations."<COMPANY>\<user>".activationPackage', 'legacyPackages.x86_64-linux.homeConfigurations."<COMPANY>\<user>".activationPackage' or 'homeConfigurations."<COMPANY>\<user>".activationPackage'
       Did you mean <COMPANY><user>?

There are two types of strings that need escaping:

• strings in Nix expressions (e.g. home.nix generated by home-manager init)
  they need backslashes before special chars
• flake URI (passed to nix build)
  they need URI's percent encoding, luckily jq supports that

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.

• Test cases updated/added. See example.

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Maintainer CC