Show intermediate update of build steps

[shashwathi]

Fixes #304

Proposed Changes

• Intermediate build steps like waiting, running.

[shashwathi]

/assign @imjasonh

[knative-metrics-robot]

The following is the coverage report on pkg/.
Say /test pull-knative-build-go-coverage to run the coverage report again

File	Old Coverage	New Coverage	Delta
pkg/builder/cluster/builder.go	80.7%	81.3%	0.6

[imjasonh]

We have a few tests that happen to specify multiple steps, e.g., test-custom-volume and test-home-volume.

It's a real limitation of our integration testing "framework" that the only way we have right now to test things against a real cluster is to send builds and expect them to succeed or fail. Once we have a more "real" framework in place (stolen from knative/serving no doubt), we can have a test that runs a multi-step build and watches to see multiple status updates.

Until then, I'm not sure this test really adds much, except that it's proof that a build that has two steps succeeds.

[shashwathi]

I completely agree with you on this.

[imjasonh]

The rest of this change looks great though, and I can't wait to get updates about the build's status while it's ongoing.

/lgtm
/approve
🎉

[knative-prow-robot]

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