build(deps): bump DeterminateSystems/nix-installer-action from 20 to 21

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](DeterminateSystems/nix-installer-action@v20...v21)

---
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>

Author: dependabot[bot]

.github/workflows/test.yml

@@ -82,7 +82,7 @@ jobs:
       - name: Check Nix flake inputs
         uses: DeterminateSystems/flake-checker-action@v12
       - name: Install Nix
-        uses: DeterminateSystems/nix-installer-action@v20
+        uses: DeterminateSystems/nix-installer-action@v21
       # according to the repository https://github.com/DeterminateSystems/magic-nix-cache
       # The Magic Nix Cache will will stop working on February 1st , 2025 (https://determinate.systems/posts/magic-nix-cache-free-tier-eol/) unless you're on GitHub Enterprise Server.
       # - name: Activate Magic Nix Cache