vimtutor: avoid warning due to Windows' lack of mktemp

This is the quickest way to avoid those warnings, and this developer deems
that good enough a work around.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

bin/vimtutor

@@ -25,7 +25,7 @@ export xx
 
 # We need a temp file for the copy.  First try using a standard command.
 tmp="${TMPDIR-/tmp}"
-TUTORCOPY=`mktemp $tmp/tutorXXXXXX || tempfile -p tutor || echo none`
+TUTORCOPY=none
 
 # If the standard commands failed then create a directory to put the copy in.
 # That is a secure way to make a temp file.