Wrap up nix-env in a way that handles conflicting, pre-installed packages

[grahamc]

Description

If an installed package contains programs or paths named like nix, nix-env -i will fail due to the conflict.

Instead of a naive nix-env -i, this PR:

• examines the default profile if it already exists
• duplicates the profile into a scratch profile, to ensure we can operate atomically on the default profile
• queries the installed packages
• checks to see if the list of packages that will be installed will conflict with already-installed paths
• if any do conflict, those conflicting packages are removed
• installs the new packages
• and finally, updates the default profile to point to the new profile.

Note that another approach could be to set the conflicting packages to a lower priority (higher numerical value.) However, that still requires most of the code here, and has the down side that it is dealing with more complexity of nix-env. The approach here -- while ... verbose ... should work in all cases.

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

[cole-h]

This may be problematic for any time we operate on profiles that may have been touched by the nix profile command:

$ nix-env --query --installed --out-path --json
error: profile '/home/vin/.local/state/nix/profiles/profile' is incompatible with 'nix-env'; please use 'nix profile' instead

(but we already have this problem, so maybe-fine to not deal with it now)

[cole-h]

After that error change, seems fine to me