incusd/instance/qemu: Don't hardcode UEFI firmware in checkFeatures #827

Job	Run time
Code	6m 53s
Client (1.20.x, ubuntu-latest)	1m 2s
Documentation	6m 48s
Client (1.20.x, macos-latest)	1m 12s
System (1.20.x, cluster, dir)	12m 27s
Client (1.20.x, windows-latest)	2m 9s
System (1.20.x, cluster, btrfs)	12m 41s
System (1.20.x, cluster, lvm)	13m 33s
System (1.20.x, cluster, zfs)	14m 22s
System (1.20.x, cluster, ceph)	16m 32s
System (1.20.x, cluster, random)	14m 6s
System (1.20.x, standalone, dir)	22m 33s
System (1.20.x, standalone, btrfs)	22m 58s
System (1.20.x, standalone, lvm)	40m 35s
System (1.20.x, standalone, zfs)	44m 3s
System (1.20.x, standalone, ceph)	1h 15m 38s
System (1.20.x, standalone, random)	36m 9s
System (stable, cluster, dir)	10m 34s
System (stable, standalone, dir)	22m 38s
System (tip, cluster, dir)	11m 17s
System (tip, standalone, dir)	22m 41s
	6h 50m 51s

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