ci: Attempt to fix by updating to use envoy-build-ubuntu

Signed-off-by: Ryan Northey <ryan@synca.io>

Author: phlax

.circleci/config.yml

@@ -1,6 +1,6 @@
 references:
   envoy-build-image: &envoy-build-image
-    envoyproxy/envoy-build:latest
+    envoyproxy/envoy-build-ubuntu:41c5a05d708972d703661b702a63ef5060125c33
 
 version: 2
 jobs:
@@ -27,4 +27,3 @@ workflows:
     jobs:
       - build
       - test
-