doc: Fix logic to find incus #171

Job	Run time
Documentation tests	10m 53s
Code tests	8m 55s
System tests (1.20.x, cluster, dir)	12m 59s
Client tests (1.20.x, ubuntu-latest)	45s
System tests (1.20.x, cluster, btrfs)	13m 45s
Client tests (1.20.x, macos-latest)	59s
System tests (1.20.x, cluster, lvm)	14m 39s
Client tests (1.20.x, windows-latest)	2m 21s
System tests (1.20.x, cluster, zfs)	15m 56s
System tests (1.20.x, cluster, ceph)	18m 39s
System tests (1.20.x, cluster, random)	13m 54s
System tests (1.20.x, standalone, dir)	25m 59s
System tests (1.20.x, standalone, btrfs)	25m 10s
System tests (1.20.x, standalone, lvm)	39m 59s
System tests (1.20.x, standalone, zfs)	42m 11s
System tests (1.20.x, standalone, ceph)	1h 15m 25s
System tests (1.20.x, standalone, random)	35m 8s
System tests (stable, cluster, dir)	12m 5s
System tests (stable, standalone, dir)	24m 56s
System tests (tip, cluster, dir)	13m 36s
System tests (tip, standalone, dir)	23m 8s
	7h 11m 22s

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