This works (if you chmod +x):
#!/usr/bin/nim e
echo 1
Tested on devel but probably works for a while.
~ cat myscript
echo "hello"
~ nim e myscript
Hint: used config file '/etc/nim/nim.cfg' [Conf]
Hint: used config file '/etc/nim/config.nims' [Conf]
Error: not a NimScript file: /home/tim/myscript
~ mv myscript myscript.nims
~ nim e myscript.nims
Hint: used config file '/etc/nim/nim.cfg' [Conf]
Hint: used config file '/etc/nim/config.nims' [Conf]
hello