Apart from the update, these are non-essential changes, but they clean things up:

• set NO_ARCH to only build package once for all architectures
• don't repeat file for ${INSTALL_*} targets
• remove build dependency of janet (now that jpm correctly requires anyway)
• remove the (put root-env :syspath ...) as the path of interpreter auto picks that path up anyway

Additionally move to latest versions.

• make test
• installed in stable/15 and made sure it still works in neovim