build-system: use a shell variable for TMPFILE in debian-snapshot

Aaron Ecay points out in
id:"1324136185-4509-1-git-send-email-aaronecay@gmail.com" that the
mktemp in

     debian-snapshot: TMPFILE := $(shell mktemp)

Is being evaluated for every target. As best I can tell, this is
because make is evaluating the right hand side, even though it is not
doing the assignment.

Of course, it isn't quite as nice to edit with the line continuations,
but it is ideomatic make.