fix(builder): hardcode STACK to cedar

This informs the official buildpacks that they should  use binaries
linked against older versions of packages.

Author: Matthew Fisher

builder/slugbuilder/builder/build.sh

@@ -63,6 +63,7 @@ su slugbuilder
 export APP_DIR="$app_dir"
 export HOME="$app_dir"
 export REQUEST_ID=$(openssl rand -base64 32)
+export STACK=cedar
 
 ## Buildpack detection