Flaky cloudbuild test due to concurrent go build ./...

[roger2hk]

The cloudbuild test is flaky from time to time because the go build ... was run concurrently after the ci-ready step.

certificate-transparency-go/scripts/presubmit.sh

Line 99 in 20f4453

go build ./...

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Sample errors

Created tree 1376086433082563736
/workspace/trillian/integration/ct_functions.sh: line 118: ./createtree: Text file busy

Created tree 2866571834323913394
/workspace/trillian/integration/ct_functions.sh: line 118: ./createtree: No such file or directory

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The fix requires some refactoring in both the scripts/presubmit.sh and cloudbuild*.yaml. Note that the coverage flag logic is inside the run_build condition.