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] <[email protected]>

Author: dependabot[bot]

.github/workflows/nix-integration-test.yml

@@ -13,5 +13,5 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@ff7abcd0c3c05ccf6adc123a8cd1fd4fb30fb493 # v4.1.1
-      - uses: DeterminateSystems/nix-installer-action@786fff0690178f1234e4e1fe9b536e94f5433196 # v20
+      - uses: DeterminateSystems/nix-installer-action@c5a866b6ab867e88becbed4467b93592bce69f8a # v21
       - run: nix flake check

.github/workflows/update-flake-lock.yml

@@ -14,6 +14,6 @@ jobs:
       - uses: actions/checkout@ff7abcd0c3c05ccf6adc123a8cd1fd4fb30fb493 # v4.1.1
         with:
           ssh-key: ${{ secrets.UPDATE_FLAKE_LOCK_SSH_KEY }}
-      - uses: DeterminateSystems/nix-installer-action@786fff0690178f1234e4e1fe9b536e94f5433196 # v20
+      - uses: DeterminateSystems/nix-installer-action@c5a866b6ab867e88becbed4467b93592bce69f8a # v21
       - name: Update flake.lock
         uses: DeterminateSystems/update-flake-lock@c5930b397a673a70ca70be06020e943aeac310a1 # v27