Remove usage of alias

[SuperSandro2000]

Without this change I get the following error when I have aliases disabled in config.nix

error:
       … while calling the 'derivationStrict' builtin

         at /derivation-internal.nix:9:12:

            8|
            9|   strict = derivationStrict drvAttrs;
             |            ^
           10|

       … while evaluating derivation 'nix-shell'
         whose name attribute is located at /nix/store/vf6skxhiqknlnqcxh8n4z2jldzzpb171-source/pkgs/stdenv/generic/make-derivation.nix:348:7

       … while evaluating attribute 'buildInputs' of derivation 'nix-shell'

         at /nix/store/vf6skxhiqknlnqcxh8n4z2jldzzpb171-source/pkgs/stdenv/generic/make-derivation.nix:395:7:

          394|       depsHostHost                = elemAt (elemAt dependencies 1) 0;
          395|       buildInputs                 = elemAt (elemAt dependencies 1) 1;
             |       ^
          396|       depsTargetTarget            = elemAt (elemAt dependencies 2) 0;

       error: undefined variable 'nixUnstable'

       at /nix/store/d7axyqzr807l6ml099jzqbjhiy36mxcn-source/flake.nix:166:13:

          165|           buildInputs = with pkgs; [
          166|             nixUnstable
             |             ^
          167|             cargo

[rvem]

I wonder if this nixUnstable is needed at all nowadays. AFAIR it was introduced back in the days when only unstable nix version had flakes support, which is no longer the case now