Lists the packages installed by `nix-env`. Moving forward, it might be
useful to run something like...
`$ nix_installed >nix-env.txt`
...and commit that to this repository a la the brew.txt file that
previously floated around this repo. For now, I'm unwilling to commit to
that solution, because I'm hoping a better alternative exists.
Perhaps this should be an alias. Still unsure why I write aliases
sometimes and functions other times. It might be worth documenting as a
principle that I can lean on.
