Disable GOPROXY for gotip

[tie]

This PR ensures that issue like #258 would fail at CI stage by disabling GOPROXY for gotip (Go 1.13).

[jhump]

Mind providing a TLDR on what these environment variables actually do?

[jhump]

I guess rebasing this (now that #258 is merged) should make the test turn green, right?

[tie]

Yes, rebasing will make the test green.

Both GOPROXY=https://proxy.golang.org,direct and GOSUMDB=sum.golang.org are defaults in Go 1.13.

• GOPROXY=direct tells Go to download modules using direct connection
  to source control servers.

  The go command can fetch modules from a proxy or connect to source control
  servers directly, according to the setting of the GOPROXY environment
  variable.
  — go help modules

• GOSUMDB=off disables the global go.sum checksum database.

Now that I think about it, GOSUMDB probably should be left intact.

[jhump]

@tie, I'm guessing Go 1.13 has dropped support for GOPATH? I ask because GO111MODULE=on is not present for tip, but it appears to be using modules nevertheless.