Attempt to fix a stream of CI failures that started on the addition of
Go modules.
I wonder if we are falling foul of this warning in the wiki
https://github.com/golang/go/wiki/Modules.
"The module cache in Go 1.11 can sometimes cause various errors,
primarily if there were previously network issues or multiple go
commands executing in parallel (see #26794, which is addressed for Go
1.12). As a troubleshooting step, you can copy $GOPATH/pkg/mod to a
backup directory (in case further investigation is warranted later), run
go clean -modcache, and then see whether the original problem persists."
Updates #21
