ci: Check the plugins found by gst-inspect

It is very easy to accidentally break the loading of a plugin. Add a
check against a pre-defined list so we notice when that happens.

We're a little lenient when the job is triggered by the gstreamer repo,
and only error out if a plugin is missing from the list. Jobs triggered
by the cerbero repo are strict and fail if the lists don't match.

This allows people to add new plugins to the gstreamer repo without
having a circular dependency on this list. We will add it to the list
the next time we make a cerbero MR.

Lines starting with `#` are treated as comments in the plugins list.
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