Heptapod CI: gitaly-test-build skip Golang build

With our prebuilt tarball or future inclusion of the prebuilds
in the base image or cache, we don't need to build the Golang
parts, but we still need bundler to do its magic for the Ruby
parts.

Building the Golang parts is still expensive in that context
because it will at the bare minimum relink everything (up to
40s in CI), and on brand new runners, it will fetch the
dependencies (cached in current user home directory) and
recompile a lot.

--HG--
branch : heptapod
7 jobs for !168 with topic/heptapod/ci-gitaly-no-build in 13 minutes and 49 seconds (queued for 1 second)
detached
Status Name Job ID Coverage
  Lint
passed rubocop #152611

00:02:00

 
  Test
passed rspec-regular 1/6 #152612

00:08:56

passed rspec-regular 2/6 #152613

00:08:36

passed rspec-regular 3/6 #152614

00:09:18

passed rspec-regular 4/6 #152615

00:10:16

passed rspec-regular 5/6 #152616

00:11:48

passed rspec-regular 6/6 #152617

00:07:50