cabal2nix: resolve crypt library to libxcrypt

[sternenseemann]

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NixOS has disabled glibc's libcrypt and is using libxcrypt now (starting with 22.11, that is): https://github.com/NixOS/nixpkgs/pull/181764

This means we'll need to resolve the crypt identifier to libxcrypt. Releases including this commit need to go into NixOS unstable (as soon as the current staging iteration has finished) and NixOS 22.11. A release including this commit may not be backported to 22.05, as it'd likely break builds there.