running packcheck a minimal complete "hello world" Haskell on NixOS

Haskell Weekly Issue 90 introduces a new Haskell "Hello World" project with full CI integration of AppVeyor and Travis.

With regard to the README the project provides a bash script packcheck.sh, which could be used for running purpose in local environment also.

The first attempt on NixOS 17.09 failed.

$ ./packcheck.sh 
bash: ./packcheck.sh: /bin/bash: bad interpreter: No such file or directory

As reported in #3 static /bin/bash in shebang doesn't work in particular environments. (For instance FreeBSD also).