configure GOROOT with go env command

[shogo82148]

goveralls doesn't use the go list command but uses the go/build package.
go list sees the GOROOT environment value, and its default value is typically /usr/local/go.
But sometimes it isn't, e.g. the go command is built from the source and customized.

So if GOROOT is not configured, get its value by running go env GOROOT.

fixes #214

[coveralls]

Coverage remained the same at 50.0% when pulling 3bf6e39 on configure-go-env into fcebdad on main.