CI: update debian image

The libc++(abi)-dev packages have been removed because one of their dependencies conflicted with one of the gstreamer dependencies. As a result, the gstreamer dependency was not installed and pkg-config was unable to satisfy the requirements for gstreamer. With the conflicting packages now gone, gstreamer is installed properly and finally the gstdecode module should be built in CI.

Amends d0c0a314.