Use nix eval instead of nix-instantiate for gomod2nix import

[jjthiessen]

Fixes #72

[adisbladis]

What are you trying to do which isn't possible with the stable CLI interface? I'm extremely reluctant to rely on the 2.4+ commands as they haven't been the most stable and have weird assumptions around flake usage that keeps changing.

[jjthiessen]

The existing single nix-instantiate call does not seem to work. However, using the nix command isn't necessary.
It seems as though running nix-instantiate --expr ...; nix-instantiate --eval --expr ... would also work.
My reasons for proposing nix eval --impure --expr ... over the two calls to nix-instantiate is that:

• It's shorter (single exec vs two)
• It doesn't produce an error when it succeeds (nix-instantiate will complain without --eval, but it appears necessary to run it without --eval first)

It may be possible to make it work with a single invocation of one of the older Nix commands (possibly adjusting the provided expression?), but I haven't really tried.
If the new commands are still deemed to be too experimental (or insufficiently available/you explicitly want to support older Nix versions), then opting for the double/back-to-back exec would work (if you don't mind the error: expression does not evaluate to a derivation (or a set or list of those) error).