Use imagePrefix (#5550)

Author: michel-laterman

.buildkite/pipeline.yml

@@ -177,7 +177,7 @@ steps:
           GO_DISTRO: "stdlib"
         agents:
           provider: "aws"
-          image: "${IMAGE_UBUNTU_X86_64_FIPS}"
+          imagePrefix: "${IMAGE_UBUNTU_X86_64_FIPS}"
           instanceType: "m5.xlarge"
         artifact_paths:
           - build/*.xml