ktest.pl: Fix uninitialized var in config-bisect.pl

ktest.pl: Fix uninitialized var in config-bisect.pl

The error path of copying the old config used the wrong variable in the
error message:

 $ mkdir /tmp/build
 $ ./tools/testing/ktest/config-bisect.pl -b /tmp/build config-good /tmp/config-bad
 $ chmod 0 /tmp/build
 $ ./tools/testing/ktest/config-bisect.pl -b /tmp/build config-good /tmp/config-bad good
 cp /tmp/build//.config config-good.tmp ... [0 seconds] FAILED!
 Use of uninitialized value $config in concatenation (.) or string at ./tools/testing/ktest/config-bisect.pl line 744.
 failed to copy  to config-good.tmp

When it should have shown:

 failed to copy /tmp/build//.config to config-good.tmp

Cc: stable@vger.kernel.org
Cc: John 'Warthog9' Hawley <warthog9@kernel.org>
Fixes: 0f0db065999cf ("ktest: Add standalone config-bisect.pl program")
Link: https://patch.msgid.link/20251203180924.6862bd26@gandalf.local.home
Reported-by: "John W. Krahn" <jwkrahn@shaw.ca>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>