chore(deps): bump DeterminateSystems/nix-installer-action from 19 to 21

Bumps [DeterminateSystems/nix-installer-action](https://github.com/determinatesystems/nix-installer-action) from 19 to 21.
- [Release notes](https://github.com/determinatesystems/nix-installer-action/releases)
- [Commits](https://github.com/determinatesystems/nix-installer-action/compare/v19...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>

.github/workflows/flake-checker.yml

@@ -16,6 +16,6 @@ jobs:
       - uses: actions/checkout@v5
         with:
           fetch-depth: 0
-      - uses: DeterminateSystems/nix-installer-action@v19
+      - uses: DeterminateSystems/nix-installer-action@v21
       - uses: DeterminateSystems/magic-nix-cache-action@v13
       - uses: DeterminateSystems/flake-checker-action@v12

.github/workflows/flake-updater.yml

@@ -13,7 +13,7 @@ jobs:
       - uses: actions/checkout@v5
         with:
           fetch-depth: 0
-      - uses: DeterminateSystems/nix-installer-action@v19
+      - uses: DeterminateSystems/nix-installer-action@v21
       - uses: DeterminateSystems/magic-nix-cache-action@v13
       - uses: DeterminateSystems/update-flake-lock@v27
         with:

.github/workflows/test-build-stream-sprout.yml

@@ -57,7 +57,7 @@ jobs:
       - name: "Checkout 🥡"
         uses: "actions/checkout@v5"
       - name: "Install Nix ❄️"
-        uses: "DeterminateSystems/nix-installer-action@v19"
+        uses: "DeterminateSystems/nix-installer-action@v21"
       - name: "Enable Magic Nix Cache 🪄"
         uses: "DeterminateSystems/magic-nix-cache-action@v13"
       - name: "Build & Test .nix ❄️"