flake: stdenv.lib -> lib

Nixpkgs on unstable has removed stdenv.lib as they've been warning for a while now. This removes the extra references to it in the flake.nix

I'm not entirely sure if this is right, but I figured it was trivial enough to give a quick try using the GH Editor while I was waiting for a job to finish

Created by  Amanda Cameron  on June 17, 2021
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