runtime: do not share underlying envs/argv array

Removes a potential data race between os.Setenv and runtime.GOROOT,
along with a bug where os.Setenv would only sometimes change the
value of runtime.GOROOT.

Change-Id: I7d2a905115c667ea6e73f349f3784a1d3e8f810d
Reviewed-on: https://go-review.googlesource.com/6611
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/export_test.go

@@ -122,3 +122,6 @@ var Open = open
 var Close = close
 var Read = read
 var Write = write
+
+func Envs() []string     { return envs }
+func SetEnvs(e []string) { envs = e }