Add REPO_URL to the buildbuddy build metadata

(cherry picked from commit cbec3b0)

Author: HoloRin

.bazelrc

@@ -5,6 +5,7 @@ build:buildbuddy --bes_backend=grpcs://cloud.buildbuddy.io
 build:buildbuddy --remote_cache=grpcs://cloud.buildbuddy.io
 build:buildbuddy --remote_timeout=1200
 build:buildbuddy --grpc_keepalive_time=30s
+build:buildbuddy --build_metadata=REPO_URL=https://github.com/rabbitmq/rabbitmq-server.git
 
 build:rbe --config=buildbuddy