Revert "cbuild: bump the qemu image partition to 3G"

This does not work, see for example the libinput test pipeline here: https://gitlab.freedesktop.org/whot/libinput/-/jobs/26244430

Something has a hard max of 2G, maybe it's the host filesystem, maybe it's some other container flag somewhere, it's unclear at this point. Either way, to avoid templates that cannot work, let's revert this change back to 2G. At least that way it fails only with out-of-space errors rather than the more confusing ones.

This reverts commit 7848fcf2.

cc @bentiss, @okias

Reverts !149 (merged) Reopens #50