vendor: bump appc/spec to v0.8.11

[squeed]

No description provided.

[alban]

	rkt_tests.go:117: Spawning command: /home/runner/rkt/builds/build-rkt-coreos/build-rkt-1.28.1+git/tmp/functional/rkt --dir=/tmp/datadir-026539693 --local-config=/tmp/localdir-257755688 --system-config=/tmp/systemdir-545940327 --user-config=/tmp/userdir-695749530 prepare --volume=test,kind=host,source=/tmp/rkt-tests-volume-data283790287 --mount volume=test,target=/mnt --volume=test,kind=empty --mount volume=test,target=/mnt/dup sha512-fabcdf4ec6781360ec343588ca1d49a2
	rkt_tests.go:359: Error: ExpectRegex didn't find regex '[0-9a-f-]{36}'.
		Output: prepare: disabling overlay support: "overlay entry not present in /proc/filesystems"

		prepare: error setting up stage0: json: error calling MarshalJSON for type schema.PodManifest: duplicate volume name "test"

	rkt_tests.go:139: rkt terminated with unexpected status 254, expected 0
		Output:
		[]

Could it be related to appc/spec@0737776?

[lucab]

@alban the failing test needs a small update, I'm on it.

[lucab]

@alban @iaguis this should be fine now, PTAL.

[squeed]

Luca's changes LGTM...

[iaguis]

lgtm