[github-action] bump DeterminateSystems/nix-installer-action

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/flake-test.yml

@@ -13,7 +13,7 @@ jobs:
       - uses: actions/checkout@v6
         with:
           fetch-depth: 0
-      - uses: DeterminateSystems/nix-installer-action@v20
+      - uses: DeterminateSystems/nix-installer-action@v21
       - uses: cachix/cachix-action@v16
         with:
           name: zz-flutter-client
@@ -31,7 +31,7 @@ jobs:
       - uses: actions/checkout@v6
         with:
           fetch-depth: 0
-      - uses: DeterminateSystems/nix-installer-action@v20
+      - uses: DeterminateSystems/nix-installer-action@v21
       - uses: cachix/cachix-action@v16
         with:
           name: zz-flutter-client