build(deps): bump cachix/install-nix-action from 15 to 26

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 15 to 26.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](cachix/install-nix-action@v15...v26)

---
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>

.github/workflows/ci.yaml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v4
-    - uses: cachix/install-nix-action@v20
+    - uses: cachix/install-nix-action@v26
     - uses: cachix/cachix-action@v12
       with:
         name: nixpkgs-update

.github/workflows/doc.yaml

@@ -14,7 +14,7 @@ jobs:
         with:
           # Nix Flakes doesn't work on shallow clones
           fetch-depth: 0
-      - uses: cachix/install-nix-action@v15
+      - uses: cachix/install-nix-action@v26
         with:
           extra_nix_config: |
             access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}

.github/workflows/flake-updates.yml

@@ -9,7 +9,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Install Nix
-        uses: cachix/install-nix-action@v25
+        uses: cachix/install-nix-action@v26
         with:
           extra_nix_config: |
             experimental-features = nix-command flakes