Detect if CI in tool/generate-config-header.sh

Author: eregon

tool/generate-config-header.sh

@@ -5,10 +5,11 @@ set -x
 
 VERSION=$(cat .ruby-version)
 
-url="$1"
-if [ -z "$url" ]; then
+if [ -n "$TRUFFLERUBY_CI" ]; then
     # The source archive, a copy from https://www.ruby-lang.org/en/downloads/
     url=$(mx urlrewrite "https://lafo.ssw.uni-linz.ac.at/pub/graal-external-deps/ruby-$VERSION.tar.gz")
+else
+    url="https://cache.ruby-lang.org/pub/ruby/${VERSION%.*}/ruby-$VERSION.tar.gz"
 fi
 
 os=$(uname -s)