Remove nix-install-package support

'nix-install-package' no longer exists in Nix 1.12.

Closes #268.

Created by  Eelco Dolstra  on January 15, 2018
ST4TUC3E5XU5SPRYPKWXCKHPCKHU4YXX7ZEZDI7PWN3WIKVEAVFQC
Dependencies
In channels
upstream
main
Change contents