nasm.nsi: in the "NASM shell", put NASM first in PATH

nasm.nsi: in the "NASM shell", put NASM first in PATH

In the "NASM shell", it makes sense for NASM to be first in the PATH,
not last.  That way if the user has more than one version of NASM
installed, and one of them is global, the "NASM shell" for any one
version will still pick up the proper version.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>