just: fix Darwin build

[austinbutler]

Motivation for this change

Looks like following the upgrade to just 0.9 it fails to build on macOS due to new dependency libiconv.

   Compiling just v0.9.0 (/private/var/folders/1s/gpt7p1x559sb7qjb_4bhq7zc75yv8y/T/nix-build-just-0.9.0.drv-0/source)
error: linking with `/nix/store/2pb1r2zgw6yiabv4v3d4j66d3vwgdd5q-clang-wrapper-7.1.0/bin/cc` failed: exit code: 1
  |
  = note: "/nix/store/2pb1r2zgw6yiabv4v3d4j66d3vwgdd5q-clang-wrapper-7.1.0/bin/cc" "-m64" "-arch" "x86_64" "-L" "/nix/store/akm781c35gr7mb14g5h0kqlk3cvyzmij-rustc-1.51.0/lib/rustlib/x86_64-apple-darwin/lib" "/private/var/folders/1s/gpt7p1x559sb7qjb_4bhq7zc75yv8y/T/nix-build-just-0.9.0.drv-0/source/target/x86_64-apple-darwin/release/deps/just.just.5y0vo6mm-cgu.1.rcgu.o" "-o" "/private/var/folders/1s/gpt7p1x559sb7qjb_4bhq7zc75yv8y/T/nix-build-just-0.9.0.drv-0/source/target/x86_64-apple-darwin/release/deps/just" "-Wl,-dead_strip" "-nodefaultlibs" "-L" "/private/var/folders/1s/gpt7p1x559sb7qjb_4bhq7zc75yv8y/T/nix-build-just-0.9.0.drv-0/source/target/x86_64-apple-darwin/release/deps" "-L" "/private/var/folders/1s/gpt7p1x559sb7qjb_4bhq7zc75yv8y/T/nix-build-just-0.9.0.drv-0/source/target/release/deps" "-L" "/nix/store/akm781c35gr7mb14g5h0kqlk3cvyzmij-rustc-1.51.0/lib/rustlib/x86_64-apple-darwin/lib" "/nix/store/akm781c35gr7mb14g5h0kqlk3cvyzmij-rustc-1.51.0/lib/rustlib/x86_64-apple-darwin/lib/libcompiler_builtins-94ac56476cd78265.rlib" "-liconv" "-lSystem" "-lresolv" "-lc" "-lm"
  = note: ld: library not found for -liconv
          clang-7: error: linker command failed with exit code 1 (use -v to see invocation)

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[austinbutler]

Result of nixpkgs-review pr 120484 run on x86_64-linux 1

[austinbutler]

Result of nixpkgs-review pr 120484 run on x86_64-darwin 1

1 package built:

• just