dl: add go1.13.3 and go1.12.12

Go 1.13.3 and 1.12.12 have been cut, add commands for them.

Change-Id: I829fcedc392163c00739aef2e9ac285bd7ec085a
Reviewed-on: https://go-review.googlesource.com/c/dl/+/201641
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

go1.12.12/main.go

@@ -0,0 +1,24 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The go1.12.12 command runs the go command from Go 1.12.12.
+//
+// To install, run:
+//
+//     $ go get golang.org/dl/go1.12.12
+//     $ go1.12.12 download
+//
+// And then use the go1.12.12 command as if it were your normal go
+// command.
+//
+// See the release notes at https://golang.org/doc/devel/release.html#go1.12.minor
+//
+// File bugs at https://golang.org/issues/new
+package main
+
+import "golang.org/dl/internal/version"
+
+func main() {
+	version.Run("go1.12.12")
+}

go1.13.3/main.go

@@ -0,0 +1,24 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The go1.13.3 command runs the go command from Go 1.13.3.
+//
+// To install, run:
+//
+//     $ go get golang.org/dl/go1.13.3
+//     $ go1.13.3 download
+//
+// And then use the go1.13.3 command as if it were your normal go
+// command.
+//
+// See the release notes at https://golang.org/doc/devel/release.html#go1.13.minor
+//
+// File bugs at https://golang.org/issues/new
+package main
+
+import "golang.org/dl/internal/version"
+
+func main() {
+	version.Run("go1.13.3")
+}