I'm fairly sure this error disappear if you attempt a rebuild now.
Earlier versions of dh-golang disabled GOCACHE, and Go 1.12 and later requires that the GOCACHE env var point to a writable directory. This is AFAIK now the case in the current dh-golang package.
If the error still occurs, can you post a bit more context of the failure? e.g., is it occurring in the build or test phase?
