This issue is being tracked here. For the next ISO coming Soon™️, the timeout has been increased to 300 seconds, which should effectively solve this issue.
In the meantime, you can edit /usr/share/calamares/modules/shellprocess_postinstall.conf
and add a timeout by changing the line with eopkg ar
to this, substituting whatever value for the timeout you want:
- command: "/usr/bin/eopkg ar Solus https://cdn.getsol.us/repo/shannon/eopkg-index.xml.xz"
timeout: 60