Set GOROOT from `go env GOROOT` if it is not set.
Set GOROOT from `go env GOROOT` if it is not set.

This should help fix any issues with the binary installed linters not
being able to find the Go root.

dnephin/gometalinter,dnephin/gometalinter,dnephin/gometalinter,alecthomas/gometalinter,alecthomas/gometalinter,alecthomas/gometalinter