[Fix] exec: -- should stop argument parsing

Co-authored-by: Vesa Vilhonen <vesa@vilhonen.com>
Co-authored-by: Sladyn Nunes <sladynnunes98@gmail.com>

test/slow/nvm exec/Running 'nvm exec' with help should not parse

@@ -0,0 +1,9 @@
+#!/bin/sh
+
+die () { echo "$@" ; exit 1; }
+
+\. ../../../nvm.sh
+
+nvm use 0.10
+
+nvm exec stable -- node --help | grep 'Usage: node [options]' || die "Help menu should have been displayed for node and not nvm"