See here:
Package fetch error: The item '/home/runner/.cache/snapcraft/download/libglapi-mesa_21.0.3-0ubuntu0.3~20.04.4_amd64.deb' could not be fetched: 404 Not Found [IP: 52.147.219.192 80]
This used to work a few days ago
It seems to be related to snapcraft 6.0, downgrading to 5.0 makes the build work again