Make license check not fail when our node version is upgraded

Created by: felixfbecker

See https://github.com/sourcegraph/sourcegraph/pull/11840

The license check seems to fail whenever the Node version is updated, preventing us from upgrading because it would poison the check on master and all branches afterwards. Could the GitHub action be made to use our .nvmrc (like Buildkite) instead of a baked-in version?