reenable gkeconnect in quickstart ci builds

[scotthart]

gkeconnect needs a release cycle to switch from using grpc connection headers to rest connection heads in the quickstart builds that rely on previous releases on google-cloud-cpp

[dbolduc]

When the time comes, we should also see if we can get it to run successfully instead of expecting errors

https://github.com/googleapis/google-cloud-cpp/blob/20b1a940c80318f8e9cec6b6a5010c5178779bd6/google/cloud/gkeconnect/CMakeLists.txt#L29-L40