Reformat code according to 'gotip' rules.

In practices adds annotations in the new syntax:
```
+//go:build !linux
 // +build !linux
```

Fixes failing gotip PASSES='fmt' check:
https://travis-ci.com/github/etcd-io/etcd/jobs/486453806

tests/integration/embed/embed_proxy_test.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build cluster_proxy
 // +build cluster_proxy
 
 // The purpose of this (empty) package is too keep following test working: