Clear GOFLAGS and use -mod=mod to fix CI build without vendor directory

[kabirbhartiRH]

The CI builder image sets GOFLAGS=-mod=vendor globally, which requires a vendor directory. Since this repo does not vendor dependencies, clear GOFLAGS and explicitly pass -mod=mod to allow Go to download dependencies at build time

[openshift-ci]

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[openshift-ci]

@kabirbhartiRH: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/build	506f76d	link	true	/test build

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