[3.0] CI: add a Debian target to compile with Qt6

Version 30:

• Update with the latest Debian sid build with VLC dependencies (see !7295 (merged)). It's using gcc 15, FFmpeg 8, libpostproc is gone and Qt 6.9.2.

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On top of !6548 (merged)

On top of !8173 (merged), !8175 (merged) and !8176 (merged).

Compilation works but using the UI is a bit broken. For example the Pause and Stop buttons don't work.