repart: use CopyFiles= instead of CopyBlocks= for ESP

CopyBlocks copies the UUID, which has to be unique per ESP, as it is
used to discover the GPT disk that we booted from

Author: bluca

mkosi.extra/usr/lib/repart.d/00-esp.conf

@@ -2,4 +2,7 @@
 
 [Partition]
 Type=esp
-CopyBlocks=auto
+Format=vfat
+CopyFiles=/boot:/
+SizeMinBytes=1G
+SizeMaxBytes=1G