I'm sure part of this should be fixed in node core, but they're just sitting on the issue (nodejs/node-v0.x-archive#3728).

PATH uses : on *nix, and ; on Windows. It could either provide just the separator, or provide some sort of push method.

Example:

shell.PATH.push('../node_modules/.bin');
// or
shell.env.PATH += shell.sep + '../node_modules/.bin';