Use custom nop image instead of busybox

[imjasonh]

Proposed Changes

• Build a custom image with Go binary that prints Build successful and exits
• Specify this image to the controller by flag
• Use this image instead of busybox running echo Build successful
• Cache this image

Release Note

[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/convert/convert.go	85.3%	85.2%	-0.1

[shashwathi]

/lgtm
/approve

More of a cleanup suggestion

• ToCRD function is used only in test, does it make more sense to have either test_helper package or move that helper function into test package?

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ImJasonH, shashwathi

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