CI: update debian unstable image

It contains more preinstalled libraries so more modules are built by default.

protoc/protobuf is also available on the system so it can be picked during
compilation.

extras/ci/gitlab-ci.yml

@@ -24,7 +24,7 @@ variables:
     VLC_WIN64_IMAGE: registry.videolan.org/vlc-debian-win64:20221214115142
     VLC_WIN_LLVM_MSVCRT_IMAGE: registry.videolan.org/vlc-debian-llvm-msvcrt:20221214101739
     VLC_WIN_LLVM_UCRT_IMAGE: registry.videolan.org/vlc-debian-llvm-ucrt:20230523085945
-    VLC_DEBIAN_IMAGE: registry.videolan.org/vlc-debian-unstable:20230823133818
+    VLC_DEBIAN_IMAGE: registry.videolan.org/vlc-debian-unstable:20230831135107
     VLC_ANDROID_IMAGE: registry.videolan.org/vlc-debian-android:20230606050714
     VLC_SNAP_IMAGE: registry.videolan.org/vlc-ubuntu-focal:20221023195550
     VLC_RASPBIAN_IMAGE: registry.videolan.org/vlc-ubuntu-raspberry:20230803092625