extras: ci: bump debian-unstable for cmake

It avoids rebuilding cmake 3.17.0, meson 0.54.2 and nasm 2.14 at every
build, speeding up the CI.

extras/ci/gitlab-ci.yml

@@ -16,7 +16,7 @@ variables:
     VLC_WIN64_IMAGE: registry.videolan.org/vlc-debian-win64:20201106143728
     VLC_WIN_LLVM_IMAGE: registry.videolan.org/vlc-debian-llvm-mingw:20201201162707
     VLC_UWP_LLVM_IMAGE: registry.videolan.org/vlc-debian-llvm-uwp:20201201150955
-    VLC_DEBIAN_IMAGE: registry.videolan.org/vlc-debian-unstable:20200229201904
+    VLC_DEBIAN_IMAGE: registry.videolan.org/vlc-debian-unstable:20200529132440
     VLC_ANDROID_IMAGE: registry.videolan.org/vlc-debian-android:20200529135226
     VLC_SNAP_IMAGE: registry.videolan.org/vlc-ubuntu-bionic:20190627090437
     VLC_RASPBIAN_IMAGE: registry.videolan.org/vlc-debian-raspbian:20200213203125