Bump cachix/install-nix-action from 20 to 21 (#20)

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 20 to 21.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](https://github.com/cachix/install-nix-action/compare/v20...v21)

---
updated-dependencies:
- dependency-name: cachix/install-nix-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/dotnet.yaml

@@ -44,7 +44,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v3
       - name: Install Nix
-        uses: cachix/install-nix-action@v20
+        uses: cachix/install-nix-action@v21
         with:
           extra_nix_config: |
             access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
@@ -57,7 +57,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v3
       - name: Install Nix
-        uses: cachix/install-nix-action@v20
+        uses: cachix/install-nix-action@v21
         with:
           extra_nix_config: |
             access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
@@ -70,7 +70,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v3
       - name: Install Nix
-        uses: cachix/install-nix-action@v20
+        uses: cachix/install-nix-action@v21
         with:
           extra_nix_config: |
             access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
@@ -83,7 +83,7 @@ jobs:
     steps:
       - uses: actions/checkout@master
       - name: Install Nix
-        uses: cachix/install-nix-action@v20
+        uses: cachix/install-nix-action@v21
         with:
           extra_nix_config: |
             access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}