bug#52869: xpra 4.3 appears to be broken

[t@fullmeta.me]

Hello guix.

Relatively fresh guix, pulled 2 days ago.

--------
$ guix describe
Generation 10	Dec 27 2021 15:36:01	(current)
  guix 9e9489f
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: 9e9489fb11ac73e51abe293235738d70365affa9
--------

Installing xpra 4.3 package as described in xorg.scm. Looks like 
it pulls a substitute with upstream log at 
https://ci.guix.gnu.org/log/sy4ar80nk2hq32axl4by7bgmns6bcwl0-xpra-4.3 
where I don't really see anything pop as suspicious

Attempting to start a session however fails:
--------
$ guix environment --ad-hoc xpra --pure -- xpra start :100
2021-12-29 09:44:20,712 Warning: cannot load cython bencode 
module: No module named 'xpra.net.bencode.cython_bencode'
Warning: using '/run/user/1000' as XDG_RUNTIME_DIR
xpra initialization error:
 start is not supported by this local installation
--------

I reported this to xpra maintainer first thinking the problem was 
there https://github.com/Xpra-org/xpra/issues/3403 but they confirmed that our build was likely incomplete. As per 
above you can already tell that cython_bencode is in fact missing 
and indeed I don't see it in the built artefact in the store, 
though it is present in the source repo. "start not supported" 
above indicates we have bigger issues, as per maintainer we 
probably don't have access to the server component. Briefly 
grepping for that warning origin, looks like we're likely failing 
the supports_server test in xpra/scripts/parsing.py:

if supports_server:
    try:
        from xpra.x11.bindings.wait_for_x_server import 
        wait_for_x_server    #@UnresolvedImport @UnusedImport
    except ImportError:
        supports_server = False


I am way out of my depth to debug this on my own, but happy to try 
and assist.

I believe the previous build of 4.2.smth worked for me, so must be 
something new. Is there an easy way to downgrade this package 
without rolling entire guix? Do I use inferior guix for that 
somehow?

Happy to follow this up with guix xpra package maintainer if only 
I could figure who that courageous soul was.

Thanks