Disable cproto tests in Modules mode for now.
Disable cproto tests in Modules mode for now.

They depend on protoc-gen-go in PATH which is not being installed
in Modules mode yet.

R=tandrii@chromium.org, 40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org
BUG=1057067

Change-Id: I32d24cf9ba0ddc5c1bae05c3c991f057c2480e89
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3018449
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go