ci: Image rebuild should also run the gst-plugins-rs tests

CC @alatiera

Otherwise it can happen that the image passes here but breaks something in gst-plugins-rs and then the CI is in a bad state for a while. Main problem here is that updating the image might require changes in gst-plugins-rs too.

added CI Enhancement labels

Is the purpose only to run the gst-plugins-rs tests inside the image built for gstreamer-rs or to also build gst-plugins-rs using the current commit of gstreamer-rs by modifying the Cargo.toml dependencies?

Both seem useful