.circleci: demonstrate how to build with old go

Author: dkegel-fastly

.circleci/config.yml

@@ -82,6 +82,7 @@ commands:
             - go-cache-v2-{{ checksum "go.mod" }}-{{ .Environment.CIRCLE_PREVIOUS_BUILD_NUM }}
             - go-cache-v2-{{ checksum "go.mod" }}
       - llvm-source-linux
+      - run: gofmt -w -r 'any -> interface{}' src
       - run: go install -tags=llvm<<parameters.llvm>> .
       - restore_cache:
           keys: