ci: Enable gst-plugins-rs documentation

Currently the cargo wrapper script depends on Python 3.8, but we have 3.7 in the old fedora we use in our image, and glib version 2.66 is required, but we currently build 2.62.

For the 3.8 deps, we can workaround with a quick shlex_join() function (excuse my style, I'm rusty in python):

def shlex_join(args):
    ret = ''
    for a in args:
        ret += ' ' + shlex.quote(a)
    return ret

For Glib, I haven't checked by we let plugins-rs be out of sync, and why we didn't sync all our subproject here. But to update, we can build manually, or update our fedora image, which is covered by !1060 (merged)