Add cabal2nix and its dependencies to my package list

Peter has retired from active cabal2nix maintenance, so is not as active
as before. I'm one of the new upstream maintainers and can act on any
issues that come up.

build-constraints.yaml

@@ -16,6 +16,11 @@ packages:
         - gopher-proxy
         - filepath-bytestring
         - download-curl
+        - cabal2nix
+        - distribution-nixpkgs
+        - hackage-db
+        - language-nix
+        - jailbreak-cabal
 
     "Profpatsch <mail@profpatsch.de> @Profpatsch":
         - error