fix: partprobe after ESP repartition to prevent stale partition table
When the install detects insufficient space on the target ESP and
repartitions to 1024MB, the kernel's in-memory partition table wasn't
being refreshed. This caused mkfs.vfat to format the old (smaller)
partition, leading to "copy failed kernel copy returned -1 bytes"
when writing the 258MB kernel.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>