fix rbenv-which with system ruby

Don't have `command -v` abort the whole script prematurely.

libexec/rbenv-which

@@ -50,7 +50,7 @@ fi
 
 if [ "$RBENV_VERSION" = "system" ]; then
   PATH="$(remove_from_path "${RBENV_ROOT}/shims")"
-  RBENV_COMMAND_PATH="$(command -v "$RBENV_COMMAND")"
+  RBENV_COMMAND_PATH="$(command -v "$RBENV_COMMAND" || true)"
 else
   RBENV_COMMAND_PATH="${RBENV_ROOT}/versions/${RBENV_VERSION}/bin/${RBENV_COMMAND}"
 fi