cmd/release: add linux-arm64

Updates golang/go#19082

Change-Id: I02cd4f0e06e1f405761967dd550cf6c7a638835a
Reviewed-on: https://go-review.googlesource.com/45913
Reviewed-by: Chris Broadfoot <cbro@golang.org>

cmd/release/release.go

@@ -179,6 +179,11 @@ var builds = []*Build{
 		Race:    true,
 		Builder: "linux-amd64",
 	},
+	{
+		OS:      "linux",
+		Arch:    "arm64",
+		Builder: "linux-arm64-packet",
+	},
 	{
 		OS:      "freebsd",
 		Arch:    "386",