Add Go modules to vendor directory

[chris-crone]

Chatting to @thaJeztah and @justincormack, for things that we expect to ship as a binary it makes more sense to include the modules as vendored code.

This also speeds up the build for the CI as we don't need to fetch anything.

[ndeloof]

This also speeds up the build for the CI as we don't need to fetch anything.

But you have to checkout vendor/, which will later include all previous versions of your dependencies with your git history, while go runtime on CI nodes could benefit go mod cache for same purpose.

[chris-crone]

It actually appears that you do need the GOFLAGS=-mod=vendor as otherwise the modules are pulled.