[Feature] Support environment variable in Profile command line

[giggio]

Describe the solution you'd like
I'd like to be able to set the command line of a profile using variables.
My particular scenario is that I have a custom copy of a Nushell installation, and I need it set to that location. I share the configuration across devices, and the username might not be the same.
In my scenario, it would be: $HOME/.nix-profile/bin/nu.

Describe alternatives you've considered
Using the hard coded path, but that has problems when the username is not the same.

Additional context
N/A.