build(deps): bump DeterminateSystems/nix-installer-action from 17 to 21

Bumps [DeterminateSystems/nix-installer-action](https://github.com/determinatesystems/nix-installer-action) from 17 to 21.
- [Release notes](https://github.com/determinatesystems/nix-installer-action/releases)
- [Commits](DeterminateSystems/nix-installer-action@v17...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/unit-tests.yml

@@ -179,7 +179,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Install Nix
-        uses: DeterminateSystems/nix-installer-action@v17
+        uses: DeterminateSystems/nix-installer-action@v21
       - name: Nix Flake Check
         run: nix build .#checks.${{ matrix.target }}.${{ matrix.checks }} -L
   flake-build:
@@ -193,6 +193,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Install Nix
-        uses: DeterminateSystems/nix-installer-action@v17
+        uses: DeterminateSystems/nix-installer-action@v21
       - name: Nix Build
         run: nix build .#packages.${{ matrix.target }}.default -L