Fix: Use mkDerivation with src instead of runCommand for test derivation

[Kranzes]

The problem with using runCommand and recreating the src directory with
lndir is that it changes the file types of individual files, they will
now be a symlink instead of a regular file. If you have a crate that tests
that a file is of regular type then it will fail inside the crate2nix derivation.

[apoelstra]

As a user with a ton of disk space I would prefer this (many of my uses of crate2nix involve nasty hacks to un-symlink files so that I can either modify them or just pass "file exists and is -f checks".

But this will use more disk space and slow things down, possibly by a lot on large repos.

[Kranzes]

I think people should filter their src's properly with something like lib.fileset. They shouldn't be copying over the entire repo to the nix store. It's not that bad IMO.