fix(flake): allow defaultPackage to evaluate

[tomeon]

by adding the missing (and in this case ignored) key/attribute-name argument to lib.mapAttrs.

Without this, trying to evaluate defaultPackage triggers the error error: value is a string while a set was expected.

Discovered in running nix flake check while working on #92.

Thanks!

[roberth]

It's a deprecated attribute, but thanks anyway!

[tomeon]

Thanks for merging this!

It's a deprecated attribute

Indeed -- kinda hard to miss given the warning message in its definition 😉 -- though I think a fuller and more salient description would be "meant-to-be-deprecated attribute whose definition broke the ability to run nix flake check". I'd be happy to tackle removing defaultPackage altogether if it would be appropriate at this point, and I suspect it probably is appropriate (or at least safe) given that defaultPackage was unusable up until you merged my changes, and nobody before me seems to have noticed.

[roberth]

Oh, yeah, it does warn about that. It's also been deprecated for a long time now. Feel free to remove it 👍