Update to nushell 0.87.1

[NotLebedev]

Great project! Learned quite a bit about nix from it and the talk. Updated this to latest (at the moment) version of nushell to make it more useful to anybody interested and myself.
Changes:

• Switched nixpkgs flake input to github:NixOS/nixpkgs/nixos-unstable (clould not figure out how to do it via flakehub)
• Replaced usages of let-env as it is deprecated now.
• Replaced source with use as it is not really necessary
• Added missing -f option when copying sources
• Fixed use of format date in example
• Fixed logging inside phases (build phase). Since this PR nushell no longer prints output of each commands automatically like bash does. Instead it behaves like conventional programming languages (like python in interpreter mode). It prints return value of last operation in repl mode only and anything explicitly printed via print command

[lucperkins]

LGTM! Thanks a lot! 💯