cmd/go: validate GOBIN early as possible

[hyangah]

GO111MODULE=on GOPATH=`mktemp -d` GOBIN=. tipgo get golang.org/x/tools/cmd/godoc
go: finding golang.org/x/tools/cmd/godoc latest
go: finding golang.org/x/tools/cmd latest
go: finding golang.org/x/tools latest
go: downloading golang.org/x/tools v0.0.0-20190131163942-067a2f313b69
go: extracting golang.org/x/tools v0.0.0-20190131163942-067a2f313b69
go: finding golang.org/x/net/context/ctxhttp latest
go: finding golang.org/x/net/context latest
go: finding golang.org/x/net latest
go: downloading golang.org/x/net v0.0.0-20190125091013-d26f9f9a57f3
go: extracting golang.org/x/net v0.0.0-20190125091013-d26f9f9a57f3
cannot install, GOBIN must be an absolute path

In most cases, the module, build caches will kick in and the work will not be completely wasted.
But it would be nice if Go detects such an obvious user error early before starting all the work.

go version devel +faf187fb8e Thu Jan 31 16:55:43 2019 +0000 linux/amd64