Some improve points maybe helpful

[ymfsing]

1. Add sha256sum in build.yaml and echo to release-notes.md

sha256sum ./guix-installer-$RELEASE_TAG.iso > ./guix-installer-$RELEASE_TAG.iso.sha256sum
echo "SHA256_ISO=$(cat ./guix-installer-$RELEASE_TAG.iso.sha256sum)" >> $GITHUB_ENV

echo $'SHA256 Checksums:\r\n\r\n```\r\n' >> release-notes.md
cat ./guix-installer-$RELEASE_TAG.iso.sha256sum >> release-notes.md
echo $'\r\n```' >> release-notes.md   

2. Add GitHub action dev-drprasad/delete-older-releases@v0.2.0 for deleting older releases

- name: Remove old release
        uses: dev-drprasad/delete-older-releases@v0.2.0
        with:
          keep_latest: 7
          delete_tags: true

3. Add nongnu substitute for descreasing build time when add linux, this idea comes from starmun-0010 / guix-installer .

[stormwatch]

...
3. Add nongnu substitute for descreasing build time when add linux, this idea comes from starmun-0010 / guix-installer .

Isn't the nonguix substitute server added here? Or this affects only the initial installation?