build.make: allow repos to use 'go mod' for vendoring
build.make: allow repos to use 'go mod' for vendoring

How a repo does vendoring is detected based on the presence of
Gopkg.toml.

The vendor check with `dep` was all done locally, but the
corresponding check for `go mod` requires network access. The check
therefore gets skipped when running in the Prow CI in situations where
we are sure that it isn't needed (for example, in a periodic job).

kubernetes-csi/csi-lib-utils,kubernetes-csi/csi-lib-utils,kubernetes-csi/external-attacher-csi,kubernetes-csi/csi-lib-utils,kubernetes-csi/csi-driver-nfs,kubernetes-csi/csi-driver-nfs,kubernetes-csi/csi-driver-nfs