chore(builder): update heroku-buildpack-go to v18

See heroku/heroku-buildpack-go@v16...v18

Author: mboersma

builder/rootfs/usr/local/src/slugbuilder/builder/install-buildpacks

@@ -39,4 +39,4 @@ download_buildpack https://github.com/heroku/heroku-buildpack-python.git
 download_buildpack https://github.com/heroku/heroku-buildpack-php.git            v82
 download_buildpack https://github.com/heroku/heroku-buildpack-clojure.git        v70
 download_buildpack https://github.com/heroku/heroku-buildpack-scala.git          v63
-download_buildpack https://github.com/heroku/heroku-buildpack-go.git             v16
+download_buildpack https://github.com/heroku/heroku-buildpack-go.git             v18