cmd/api: don't depend on os/user or USER to check api

The -nocgo builder failed because it has cgo disabled
as well as no USER environment variable:
http://build.golang.org/log/2250abb82f5022b72a12997b8ff89fcdeff094c9

# Checking API compatibility.
Error getting current user: user: Current not implemented on linux/amd64
exit status 1

Don't require the environment variable here.

LGTM=minux
R=dave, adg, minux
CC=golang-codereviews
https://golang.org/cl/140290043

src/cmd/api/run.go

@@ -98,11 +98,9 @@ func prepGoPath() string {
 	if err == nil {
 		username = u.Username
 	} else {
-		// Only need to handle Unix here, as Windows's os/user uses
-		// native syscall and should work fine without cgo.
 		username = os.Getenv("USER")
 		if username == "" {
-			log.Fatalf("Error getting current user: %v", err)
+			username = "nobody"
 		}
 	}