Fix knative installation for Gardener

[vlerenc]

Missing manifest, cc @vasu1124.

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[vlerenc]

/assign @mattmoor

Could you please take a look (the command was suggested automatically).

[samodell]

This is the same issue going on with #944 and #945 -- in theory, you shouldn't need to explicitly install the in-memory channel provisioner if you install eventing from https://github.com/knative/eventing/releases/download/v0.4.0/release.yaml, but in practice people are getting errors without it. We're investigating this.

/hold

[samodell]

Verified this morning that you don't have to explicitly install the in-memory channel provisioner. If install fails on the first attempt, just re-rerun the install commands and it will succeed on second attempt.