.github: Bump DeterminateSystems/nix-installer-action from 20 to 21 (#18453)

Bumps [DeterminateSystems/nix-installer-action](https://github.com/determinatesystems/nix-installer-action) from 20 to 21.
- [Release notes](https://github.com/determinatesystems/nix-installer-action/releases)
- [Commits](https://github.com/determinatesystems/nix-installer-action/compare/786fff0690178f1234e4e1fe9b536e94f5433196...c5a866b6ab867e88becbed4467b93592bce69f8a)

---
updated-dependencies:
- dependency-name: DeterminateSystems/nix-installer-action
  dependency-version: '21'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/flakehub-publish-tagged.yml

@@ -20,7 +20,7 @@ jobs:
       - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
         with:
           ref: "${{ (inputs.tag != null) && format('refs/tags/{0}', inputs.tag) || '' }}"
-      - uses: DeterminateSystems/nix-installer-action@786fff0690178f1234e4e1fe9b536e94f5433196 # v20
+      - uses: DeterminateSystems/nix-installer-action@c5a866b6ab867e88becbed4467b93592bce69f8a # v21
       - uses: DeterminateSystems/flakehub-push@71f57208810a5d299fc6545350981de98fdbc860 # v6
         with:
           visibility: "public"