nixpkgs-lint: don't find license with new syntax

[davidak]

for example this has a correct license:

https://github.com/NixOS/nixpkgs/blob/master/pkgs/tools/system/ioping/default.nix#L17

but nixpkgs-lint don't recognize it:

$ nixpkgs-lint -f /home/davidak/nixpkgs/ -p ioping
=== Package meta information ===

ioping: Lacks a license

=== Package name collisions ===

=== Bottom line ===
Number of packages: 1
Number of bad names: 0
Number of missing maintainers: 0
Number of missing platforms: 0
Number of missing licenses: 1
Number of missing descriptions: 0
Number of bad descriptions: 0
Number of name collisions: 0

[pSub]

Fixed with NixOS/nix#712 in the next version of nix.

[vcunat]

Thanks!

[vcunat]

Next step: have a bot using it on PRs :-)

[davidak]

@vcunat great idea. remind if basic meta-data is missing. would you create an issue for that?