Use $(NM) not nm in tst-cancel-wrappers

[Joseph S. Myers]

On Thu, 18 Oct 2012, Carlos O'Donell wrote:

> I would rather this variable be called something other
> than nm, such that it clearly fails if the "$" is left
> off by accident.

I don't think the "$" being left off by accident is a significantly more 
plausible mistake than anything else someone might do wrong when editing 
this script.

-- 
Joseph S. Myers
joseph@codesourcery.com