benchmark: disable only the ESLint rule needing it

In the spread-assign.js benchmark file, all ESLint rules are disabled
for a line where only no-unused-vars needs to be disabled. This change
makes it so that the remaining rules are still applied to the line.

PR-URL: https://github.com/nodejs/node/pull/21133


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>