It also seems to create some files in /root/.cache
This can be fixed by setting the GOCACHE variable. I'm thinking about setting this to the build directory the slackbuilds creates, so it will be cleaned up if the slackbuild is re-run.
It looks like all required modules are already included in the source, so we might as well use them by disabling the new go modules system with GO111MODULE=off as mentioned.
This also will stop go from creating any files in /root/go.
I will do some more testing and if all goes well I'll update the slackbuild soon