build: fix step name in GitHub Actions workflow

The CI workflow doesn't use the matrix strategy so there is no
`${{ matrix.node-version }}` variable to use in the `Use Node.js`
step names.

PR-URL: https://github.com/nodejs/node/pull/31323


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>