Add upx to goreleaser builds

Reducing the size of the final binaries, except for darwin, where the
packed binaries failed to run.

scripts/upx.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+set -ex
+
+for dist_dir in ./dist/ctrld*; do
+  upx --brute "${dist_dir}/ctrld"
+done