treewide: don't hardcode /nix/store (no rebuilds changes)

[Artturin]

improve experience for other store locations

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.05 Release Notes (or backporting 22.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[Artturin]

Unnecessary for this pr but here's a useful command(s)

sudo mkdir -p /test-nix/var/nix && sudo chown --recursive $USER:$USER /test-nix

$(nix build --impure --expr 'with import <nixpkgs> { }; (nix.override { storeDir = "/test-nix/store"; stateDir = "/test-nix/var"; }).out' --no-link --print-out-paths)/bin/nix build ".#bash"

[Artturin]

Thanks @wegank