elvish: a small fixup (#54531)

* Elvish: a small fixup

Fixing version variable in order to properly download the source code.
Also, some minor stylistical modifications and additions.

* elvish: use pname

* Update pkgs/shells/elvish/default.nix

Use `pname` instead of `name`.

Co-Authored-By: AndersonTorres <torres.anderson.85@protonmail.com>