[LTP] [PATCH 3/4] ci: add ci-patchwork-trigger workflow

[Andrea Cervesato]

Apparently this patch is totally missing from patchwork [1]. It's 
clearly a bug of the platform and it might affect our automate testing, 
since the "series-completed" event is triggered only when all the 
patch-series are received and it's the one checked by our scripts. I'm 
scared we will need to test each single patch via "b4 shazam" checking 
"patch-completed"

Also b4 complains:

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---
   ✗ [PATCH 1/4] ci: install dependences for patchwork-ci script
     ✗ No key: ed25519/andrea.cervesato@suse.com
     ✗ BADSIG: DKIM/suse.de
     ✗ BADSIG: DKIM/suse.de
     ✗ BADSIG: DKIM/suse.de
     ✗ BADSIG: DKIM/suse.de
   ✗ [PATCH 2/4] ci: add patchwork-ci script
     ✗ No key: ed25519/andrea.cervesato@suse.com
     ✗ BADSIG: DKIM/suse.de
     ✗ BADSIG: DKIM/suse.de
     ✗ BADSIG: DKIM/suse.de
     ✗ BADSIG: DKIM/suse.de

------------------>  ERROR: missing [3/4]! <--------------

   ✗ [PATCH 4/4] ci: apply patchwork series in ci-docker-build workflow
     ✗ No key: ed25519/andrea.cervesato@suse.com
     ✗ BADSIG: DKIM/suse.de
     ✗ BADSIG: DKIM/suse.de
     ✗ BADSIG: DKIM/suse.de
     ✗ BADSIG: DKIM/suse.de

This is so frustrating after days of work :-)

[1] 
https://patchwork.ozlabs.org/project/ltp/patch/20250410-patchwork_ci-v1-4-def85825f46a@suse.com/

- Andrea