stdenv/check-meta: Fix error message for disallowed unfree packages

Nixpkgs tries to print a helpful message when it blocks unfree packages,
but the suggestion is subtly broken. The predicate only matches on the
package's name, but the suggestion includes the full name-version pair.

Fixed by formatting the message with the same function as the predicate.

This issue arises because check-meta defines its own local getName with
semantics divergent from lib.getName. The former includes the version,
the latter does not.

Example Before:

    Alternatively you can configure a predicate to allow specific packages:
      { nixpkgs.config.allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
          "obsidian-1.5.12"
        ];
      }

Example After:

    Alternatively you can configure a predicate to allow specific packages:
      { nixpkgs.config.allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
          "obsidian"
        ];
      }

Fixes #303116