icr: do not wrap crystal so we can use one from the environment

[peterhoeg]

Motivation for this change

Further to #61156, do not wrap crystal so icr will work with whatever crystal is currently available in the environment.

This means that installing icr on its own isn't enough to make it work - you need to have crystal available, either in the global environment, the user environment or using something like direnv (recommended).

Cc: @c0bw3b

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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