misc/dashboard/builder: add environment variables for Plan 9

We require $objtype in make.rc and rc needs $path for finding commands.

Also include $cputype which we may use in the future.

R=golang-dev, minux.ma, r
CC=golang-dev
https://golang.org/cl/9905043

misc/dashboard/builder/main.go

@@ -31,11 +31,18 @@ const (
 // These variables are copied from the gobuilder's environment
 // to the envv of its subprocesses.
 var extraEnv = []string{
-	"CC",
 	"GOARM",
+
+	// For Unix derivatives.
+	"CC",
 	"PATH",
 	"TMPDIR",
 	"USER",
+
+	// For Plan 9.
+	"objtype",
+	"cputype",
+	"path",
 }
 
 type Builder struct {