cmd/go: errors about netrc need go prefix

[rsc]

I ran 'go test' in a subprocess with no environment at all (accidentally), and it failed with

could not parse netrc (GOAUTH=netrc): $HOME is not defined

The usual convention is that calls to base.Fatalf need to provide a 'go: ' prefix in the error.
The ones in auth should be fixed.

/cc @matloob @samthanawalla

[gopherbot]

Change https://go.dev/cl/631535 mentions this issue: cmd/go: add go prefix in base.Fatalf

[cuishuang]

The error message here comes from

go/src/cmd/go/internal/auth/netrc.go

Line 77 in b68f8ca

dir, err := os.UserHomeDir()

If no environment variables are set, os.UserHomeDir() will return an error. I wonder if we need to add a logic here to get the result of cd ~ && pwd and try to find the .netrc file in that path.