build: Force module mode and using only the vendor directory

There are a handful of scripts that were making `go run` calls without
the required environment vars setup force go to use modules and only
use the vendor directory.

For golang 1.11/12 if OPA was in a path outside of the GOPATH it would
switch into module mode automatically (by default) and then try to
fetch packages and stuff remotely rather than use the vendor dir.

For golang 1.13+ any directory for OPA would (by default) result in
trying to use the remote modules rather than vendor dir.

Fixes: #2063
Signed-off-by: Patrick East <east.patrick@gmail.com>