Stalls on mesa build

When trying to add salsa-ci for mesa it stalls (reproducibly) at ninja's 1193/2398 (after <30 min).

https://salsa.debian.org/agx/mesa/-/jobs/434370

(i canceled the build after letting it sit there for a while to not block resources).

Once it gets terminated it fails to upload the cache file (which would speed up subsequent builds):

https://salsa.debian.org/agx/mesa/-/jobs/434370

Is there anything i can do to help the builds pass? My plan was to disable time/resouce intensive tasks like repotest for the time being anyway but having builds / piuparts / lintian would already go a long way.

I saw similar behaviour with libvirt a couple of weeks ago when trying to set up ci there.