There was some debate about this, and wether or not tidy should be run
when vendoring, but without this, validation failed after running
`make vendor`, and manually doing a `go mod tidy` is complicated
(due to the `vendor.mod`, and because the cache is not inside the
build-cache, not on the host).
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
