In a couple places in tests we execute a child "go build" to make a helper
program. Now that we're running in module mode, "go build" will normally
default to downloading and caching dependencies, which we don't want
because we're still using vendoring for the moment.
Therefore we need to instruct these child builds to use vendoring too,
avoiding the need to download all of the dependencies and ensuring that
we'll be building with the same dependencies that we'd use for a normal
build.
Martin Atkins