v3.1.1 — bootc-boot-smoke --rootfs flag

Single-fix patch. Tier 4 verify-downstream from v3.1.0 surfaced that
bootc-boot-smoke didn't pass --rootfs to BIB, and the
bootc.diskimage-builder.rootfs label on stable tags isn't reliable.
New rootfs input (default xfs) matches installer-anaconda-iso.

This is the v3.x line settling: with v3.1.0 we proved Tier 4 fires
correctly and validates against live consumers; v3.1.1 fixes the
first real bug Tier 4 caught.