Python generate.sh should be able to be executed from anywhere

[beneboy]

The python generate.sh (and possibly other scripts) use a relative path. They should read the path of the current sh script and use that to generate the path to the nix env.

The generate.sh script failed until I was in the correct directory:

nix/pkgs/development/python-modules/generate.sh 
error: opening file '/Users/default.nix': No such file or directory

python-modules/generate.sh            
error: opening file '/Users/ben/Documents/stencila/default.nix': No such file or directory

./generate.sh 
unpacking 'https://github.com/NixOS/nixpkgs/archive/18.09.tar.gz'...
these derivations will be built: