switch hackage category from System to Nix

Author: sorki

hnix-store-core/hnix-store-core.cabal

@@ -11,7 +11,7 @@ license-file:        LICENSE
 author:              Shea Levy
 maintainer:          [email protected]
 copyright:           2018 Shea Levy
-category:            System
+category:            Nix
 build-type:          Simple
 extra-source-files:  ChangeLog.md, README.md
 cabal-version:       >=1.10

hnix-store-remote/hnix-store-remote.cabal

@@ -8,7 +8,7 @@ license-file:        LICENSE
 author:              Richard Marko
 maintainer:          [email protected]
 copyright:           2018 Richard Marko
-category:            System
+category:            Nix
 build-type:          Simple
 extra-source-files:  README.md
 cabal-version:       >=1.10