.github: do not push to tag on push to main - #8

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683, DeterminateSystems/nix-installer-action@21a544727d0c62386e78b4befe52d19ad12692e3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/