scripts: fix get-protoc script to work directly after a clean
scripts: fix get-protoc script to work directly after a clean

Right now `make grpc` only works if a user hasn't run a `make clean`.
Fix this.

coreos/dex,chub/coreos-dex,holgerkoser/dex,holgerkoser/dex,cjyar/dex,ericchiang/dex,cjyar/dex,wolfeidau/dex,jwmaag/dex,circleci/dex,rithujohn191/dex,coreos/dex,circleci/dex,xeonx/dex,rithujohn191/dex,holgerkoser/dex,chub/coreos-dex,Calpicow/dex,jwmaag/dex,xeonx/dex,chub/coreos-dex,jwmaag/dex,ericchiang/dex,ericchiang/dex,xeonx/dex,wolfeidau/dex,circleci/dex,rithujohn191/dex,cjyar/dex,wolfeidau/dex,Calpicow/dex,Calpicow/dex,coreos/dex