Motivation for this change

sloccount scripts are not wrapped so invoking sloccount fails because it doesn't find its companion scripts and binaries:

$ sloccount .
/nix/store/ankhdwbyalcvh3xm3kswzm57wyy9cysm-sloccount-2.26/bin/sloccount: line 215: make_filelists: command not found

I avoid wrapping the compiled binaries (c_count, java_count and jsp_count for example) because they don't seem to need it. I can change this to wrap everything as well if it's preferable.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions (CentOS 7.4 single user)
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• *Tested execution of a number of scripts and binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

* I also ran sloccount on the sloccount provided "testcode" as well, which exercises a number of languages, without problems.