Add Ubuntu 26.04 3.11 Qt6 builder

[haakov]

Haven't finished testing this one yet, I just used one of the older builders as a blueprint and added dependencies from apt showsrc gnuradio | grep '^Build-Depends'. The .deb installs may not be the optimal way to install Qwt for Qt6.

[haakov]

From CMake output, building the haakov/qt5to6 branch:

-- Python checking for PyQt6 - found
-- Checking for module 'Qt6Qwt6'
--   Package 'Qt5Widgets', required by 'Qt6Qwt6', not found
Package 'Qt5Concurrent', required by 'Qt6Qwt6', not found
Package 'Qt5PrintSupport', required by 'Qt6Qwt6', not found
Package 'Qt5Svg', required by 'Qt6Qwt6', not found
Package 'Qt5OpenGL', required by 'Qt6Qwt6', not found
-- QWT Version: 6.3.0
-- QWT_FOUND, qwt_FOUND=, Qwt_FOUND=TRUE
--
-- Configuring gr-qtgui support...
--   Component Dependency ENABLE_GNURADIO_RUNTIME = ON
--   Component Dependency ENABLE_GR_FFT = ON
--   Component Dependency ENABLE_GR_FILTER = ON
--   External Dependency QT_FOUND = 1
--   External Dependency QWT_FOUND = TRUE
--   External Dependency PYTHONLIBS_FOUND = TRUE
--   External Dependency PYQT_FOUND = TRUE
--   Enabling gr-qtgui support.
--   Override with -DENABLE_GR_QTGUI=ON/OFF
-- gr-qtgui: Qt6 OpenGL/OpenGLWidgets found, fosphor/rfnoc_f15 display enabled
-- Assigning gnuradio-qtgui to component gr-qtgui