fortify: Rename temporary file to match ignore pattern

fortify: Rename temporary file to match ignore pattern

test_fortify.sh uses a temporary file that might appear as untracked
file in some rare sitations.  Rename it to match one of top-level's
gitignore patterns.

Reported-by: WangYuli <wangyuli@aosc.io>
Closes: https://lore.kernel.org/linux-kbuild/20251112114725.287349-1-wangyuli@aosc.io/
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: WangYuli <wangyuli@aosc.io>
Signed-off-by: Nicolas Schier <nsc@kernel.org>
Link: https://patch.msgid.link/20260114-fortify-improve-handling-of-tempfile-v2-1-63b86c4dbd0e@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>