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: [e-mail]
Cc: John 'Warthog9' Hawley <[e-mail]>
Fixes: 0f0db065999cf ("ktest: Add standalone config-bisect.pl program")
Link: https://patch.msgid.link/[e-mail]
Reported-by: "John W. Krahn" <[e-mail]>
Signed-off-by: Steven Rostedt <[e-mail]>

tools/testing/ktest/config-bisect.pl

···
 	die "Can not find file $bad\n";
     }
     if ($val eq "good") {
-	run_command "cp $output_config $good" or die "failed to copy $config to $good\n";
+	run_command "cp $output_config $good" or die "failed to copy $output_config to $good\n";
     } elsif ($val eq "bad") {
-	run_command "cp $output_config $bad" or die "failed to copy $config to $bad\n";
+	run_command "cp $output_config $bad" or die "failed to copy $output_config to $bad\n";
     }
 }