nix-prefetch-git broken (maybe macos only)

[will]

nix-prefetch-git is failing with error: path '/tmp' is a symlink, which then causes json parsing to fail:

❯ crystal2nix
Initialized empty Git repository in /private/tmp/git-checkout-tmp-UCUG3iTb/backtracer.cr/.git/
From https://github.com/sija/backtracer.cr
 * tag               v1.2.2     -> FETCH_HEAD
Switched to a new branch 'fetchgit'
removing `.git'...
error: path '/tmp' is a symlink
Unhandled exception: Expected BeginObject but was EOF at line 1, column 1
  parsing Crystal2Nix::PrefetchJSON at line 0, column 0 (JSON::SerializableError)
  from /nix/store/fpnllrcmwbhpj2l3njin35jjb2f7jfdb-crystal2nix-0.3.0/bin/.crystal2nix-wrapped in 'raise<JSON::SerializableError>:NoReturn'
  from /nix/store/fpnllrcmwbhpj2l3njin35jjb2f7jfdb-crystal2nix-0.3.0/bin/.crystal2nix-wrapped in 'Crystal2Nix::Worker#run:Nil'
  from /nix/store/fpnllrcmwbhpj2l3njin35jjb2f7jfdb-crystal2nix-0.3.0/bin/.crystal2nix-wrapped in '__crystal_main'
  from /nix/store/fpnllrcmwbhpj2l3njin35jjb2f7jfdb-crystal2nix-0.3.0/bin/.crystal2nix-wrapped in 'main'
Caused by: Expected BeginObject but was EOF at line 1, column 1 (JSON::ParseException)
  from /nix/store/fpnllrcmwbhpj2l3njin35jjb2f7jfdb-crystal2nix-0.3.0/bin/.crystal2nix-wrapped in 'raise<JSON::ParseException>:NoReturn'
  from /nix/store/fpnllrcmwbhpj2l3njin35jjb2f7jfdb-crystal2nix-0.3.0/bin/.crystal2nix-wrapped in 'JSON::PullParser#raise<String>:NoReturn'
  from /nix/store/fpnllrcmwbhpj2l3njin35jjb2f7jfdb-crystal2nix-0.3.0/bin/.crystal2nix-wrapped in 'Crystal2Nix::Worker#run:Nil'
  from /nix/store/fpnllrcmwbhpj2l3njin35jjb2f7jfdb-crystal2nix-0.3.0/bin/.crystal2nix-wrapped in '__crystal_main'
  from /nix/store/fpnllrcmwbhpj2l3njin35jjb2f7jfdb-crystal2nix-0.3.0/bin/.crystal2nix-wrapped in 'main'

I found NixOS/nixpkgs#358685 and NixOS/nix#11941 which seem releated, but mention that this may be a macos only problem

[Mic92]

Nixpkgs now has a workaround for that.