improve default npm path handling

[krgn]

I added a test to check whether npm exists in /usr/bin as a standard location in addition to the existing default /usr/local/bin. Also, I changed the shebang of build.sh such that it uses /usr/bin/env bash instead of hard-coding the path to bash (which does not work on systems like NixOS).

[TheAngryByrd]

Would it make more sense to try to use what's on the environments path first? Then maybe default to these?

[krgn]

Yes, that is really the better way to do it. Gimme a sec

[krgn]

It now attempts to extract the path via which on Unix systems, and sets the default to /usr/bin/npm if that is not possible for some reason.

[forki]

cool thanks.