internal/testenv: add Builder, to report builder name

This works after golang.org/cl/13120 is running on the
coordinator (maybe it already is).

Change-Id: I4053d8e2f32fafd47b927203a6f66d5858e23376
Reviewed-on: https://go-review.googlesource.com/13165Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/internal/testenv/testenv.go

@@ -11,11 +11,20 @@
 package testenv
 
 import (
+	"os"
 	"runtime"
 	"strings"
 	"testing"
 )
 
+// Builder reports the name of the builder running this test
+// (for example, "linux-amd64" or "windows-386-gce").
+// If the test is not running on the build infrastructure,
+// Builder returns the empty string.
+func Builder() string {
+	return os.Getenv("GO_BUILDER_NAME")
+}
+
 // HasGoBuild reports whether the current system can build programs with ``go build''
 // and then run them with os.StartProcess or exec.Command.
 func HasGoBuild() bool {