SCRIPTS: announce-release: add URL of dev packages

This is the shortened URL of the nightly builds maintained by William,
let's have them in announce messages.
diff --git a/scripts/announce-release b/scripts/announce-release
index 2510788..37e2ac4 100755
--- a/scripts/announce-release
+++ b/scripts/announce-release
@@ -223,6 +223,7 @@
  echo "   Pending bugs     : http://www.haproxy.org/l/pending-bugs"
  echo "   Reviewed bugs    : http://www.haproxy.org/l/reviewed-bugs"
  echo "   Code reports     : http://www.haproxy.org/l/code-reports"
+ echo "   Latest builds    : http://www.haproxy.org/l/dev-packages"
 ) >> "$OUTPUT"
 
 # sign