Pin sources release to last known good

[grantr]

Trying to unblock tests while we track down the failure in sources.

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[evankanderson]

We should consider using a release here now that we have it, rather than the latest nightly.

/lgtm

[grantr]

I'm fine with using the versioned release. FWIW the tests use the nightly release of serving:

eventing/vendor/github.com/knative/test-infra/scripts/library.sh

Line 28 in 7561571

readonly KNATIVE_SERVING_RELEASE=https://storage.googleapis.com/knative-nightly/serving/latest/release.yaml

@vaikas-google thoughts?

[vaikas]

I think it depends on what we're testing. I think we should know asap if say eventing-sources are not going to work with eventing anymore.
So, we should probably from Eventing test against a known good.
eventing-sources should probably test against the head to know if something there will break against eventing.