[REG 6.0.2 -> 7.0.0] gdb not autodetected

1. Have qmake and gdb in PATH.
   I have my own build of Qt 6.2.3 and gdb from Ubuntu's repositories.

   qtrob@rob-desktop:~$ which qmake; which gdb
   /home/qtrob/dev/g++-9.3.0/qt-6.2.3-nowebengine-r/qtbase/bin/qmake
   /usr/bin/gdb
   

2. Start Creator with empty settings.
3. Go to "Tools" -> "Options..." -> "Kits" -> "Kits".
   The kit "Desktop (default)" has a warning icon because it has no debugger set up.

gdb from PATH should have been autodetected. Creator built from 6.0 branch finds it.