cmd/update.sh: define conditions for unsetting HOMEBREW_RUBY_PATH

Library/Homebrew/cmd/update.sh

@@ -599,6 +599,13 @@ EOS
         -d "$HOMEBREW_LIBRARY/LinkedKegs" ||
         (-n "$HOMEBREW_DEVELOPER" && -z "$HOMEBREW_UPDATE_PREINSTALL") ]]
   then
+    if [[ -n $HOMEBREW_FORCE_VENDOR_RUBY &&
+          -x $HOMEBREW_LIBRARY/Homebrew/vendor/portable-ruby/current/bin/ruby ]]
+    then
+      export HOMEBREW_RUBY_PATH="$HOMEBREW_LIBRARY/Homebrew/vendor/portable-ruby/current/bin/ruby"
+    elif [[ -z $HOMEBREW_DEVELOPER ]]; then
+      unset HOMEBREW_RUBY_PATH
+    fi
     brew update-report "$@"
     return $?
   elif [[ -z "$HOMEBREW_UPDATE_PREINSTALL" ]]