Add support for URLs or paths in --nix-package-url and --extra-conf

[Hoverbear]

Description

During testing I quite often want to pass file paths to --nix-package-url, the file:/// URL bits always trip me up and require me to think about my cwd etc. This PR tidies up some related code and adds a similar functionality to the --extra-conf such that I can pass something that looks like a URL or file path and have it just work.

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