inklecate: use dotnet 3.1 to compile

pkgs/development/compilers/inklecate/default.nix

···
   executables = [ "inklecate" ];
 
   dotnet-runtime = dotnetCorePackages.runtime_3_1;
+  dotnet-sdk = dotnetCorePackages.sdk_3_1;
 
   meta = with lib; {
     description = "Compiler for ink, inkle's scripting language";