ProvisionDeterminateNixd: cleanup /etc/nix and /etc/nix/nix.conf

[cole-h]

determinate-nixd fully manages /etc/nix/nix.conf, so it's safe to remove that file unconditionally. If there are no other files in /etc/nix, then we'll also remove that directory; otherwise, we leave it alone.

If we don't clean this up, it prevents uninstall-reinstalls from working.

Description

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

Validating with install.determinate.systems

If a maintainer has added the upload to s3 label to this PR, it will become available for installation via install.determinate.systems:

curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix/pr/$PR_NUMBER | sh -s -- install