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 package now gone, gstreamer is installed properly and the Debian CI job is now able to build the gstdecode module.

Amends d0c0a314.