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@786fff0...c5a866b)

---
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]

action.yml

@@ -144,7 +144,7 @@ runs:
 
     - name: Install nix
       if: inputs.skip-nix-installation == 'false'
-      uses: DeterminateSystems/nix-installer-action@786fff0690178f1234e4e1fe9b536e94f5433196  # v20
+      uses: DeterminateSystems/nix-installer-action@c5a866b6ab867e88becbed4467b93592bce69f8a  # v21
       with:
         logger: pretty
         extra-conf: experimental-features = ca-derivations fetch-closure