lxd-to-incus: Clear cache on migration #1115

Job	Run time
Code	7m 20s
Client (1.20.x, ubuntu-latest)	1m 2s
Documentation	6m 20s
Client (1.20.x, macos-latest)	1m 14s
Client (1.20.x, windows-latest)	6m 4s
System (1.20.x, cluster, dir)	12m 29s
System (1.20.x, cluster, btrfs)	12m 40s
System (1.20.x, cluster, lvm)	13m 12s
System (1.20.x, cluster, zfs)	13m 51s
System (1.20.x, cluster, ceph)	16m 12s
System (1.20.x, cluster, random)	12m 59s
System (1.20.x, standalone, dir)	22m 18s
System (1.20.x, standalone, btrfs)	21m 50s
System (1.20.x, standalone, lvm)	41m 27s
System (1.20.x, standalone, zfs)	43m 5s
System (1.20.x, standalone, ceph)	1h 15m 26s
System (1.20.x, standalone, random)	37m 33s
System (stable, cluster, dir)	12m 23s
System (stable, standalone, dir)	22m 54s
System (tip, cluster, dir)	11m 18s
System (tip, standalone, dir)	21m 40s
	6h 53m 17s

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