cmd/go: TestDefaultGOPATH failures

```
#!watchflakes
default <- pkg == "cmd/go" && test == "TestDefaultGOPATH"
```

Issue created automatically to collect these failures.

Example ([log](https://ci.chromium.org/b/8695331004038608561)):

=== RUN TestDefaultGOPATH
=== PAUSE TestDefaultGOPATH
=== CONT TestDefaultGOPATH
go_test.go:1235: running testgo [env GOPATH]
go_test.go:1235: standard output:
go_test.go:1235: /Users/swarming/.swarming/w/ir/x/t/cmd-go-test-765590296/gotest3021853762/home/go

go_test.go:1239: running testgo [env GOPATH]
go_test.go:1239: standard output:
go_test.go:1239:

go_test.go:1239: go [env GOPATH] failed unexpectedly in /Users/swarming/.swarming/w/ir/x/w/goroot/src/cmd/go: exec: WaitDelay expired before I/O complete
--- FAIL: TestDefaultGOPATH (64.09s)

— [watchflakes](https://go.dev/wiki/Watchflakes)

Start with TestDefaultGOPATH in cmd/go, using the failure log and the references to go_test.go:1235 and go_test.go:1239. Reproduce the failure and trace the GOPATH environment checks; done means the test completes without the WaitDelay failure.

Written by the indexing model from the issue text.