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

scripts/fix.sh

@@ -5,9 +5,15 @@ set -e
 source ./scripts/test_lib.sh
 source ./scripts/updatebom.sh
 
+# To fix according to newer version of go:
+# go get golang.org/dl/gotip
+# gotip download
+# GO_CMD="gotip"
+GO_CMD="go"
+
 function mod_tidy_fix {
   run rm ./go.sum
-  run go mod tidy || return 2
+  run ${GO_CMD} mod tidy || return 2
 }
 
 function bash_ws_fix {
@@ -23,7 +29,7 @@ function bash_ws_fix {
 log_callout -e "\\nFixing etcd code for you...\\n"
 
 run_for_modules mod_tidy_fix || exit 2
-run_for_modules run go fmt || exit 2
+run_for_modules run ${GO_CMD} fmt || exit 2
 run_for_module tests bom_fix || exit 2
 bash_ws_fix || exit 2