internal: try3 more go get

This helps avoid failures caused by network hiccups.

Change-Id: I9e5a2de6b83769bab691aabbbf860dd662845be3
Reviewed-on: https://code-review.googlesource.com/c/35590
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Eno Compton <enocom@google.com>

internal/kokoro/vet.sh

@@ -14,7 +14,9 @@ fi
 
 pwd
 
-go get -u \
+try3() { eval "$*" || eval "$*" || eval "$*"; }
+
+try3 go get -u \
   golang.org/x/lint/golint \
   golang.org/x/tools/cmd/goimports \
   golang.org/x/lint/golint \