go_repository: respect proxy and sumdb privacy environment variables (#580)
go_repository: respect proxy and sumdb privacy environment variables (#580)

* GOPRIVATE, GONOPROXY, GOSUMDB, and GONOSUMDB are now passed through
  to gazelle when invoked by go_repository.
* GOSUMDB is set to "off" for fetch_repo. In module mode, the zip sum
  is a mandatory attribute of go_repository, and we don't need to look
  it up.

Updates #579
bazelbuild/bazel-gazelle,bazelbuild/bazel-gazelle