build: bump cachix/install-nix-action from 25 to 26

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 25 to 26.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](cachix/install-nix-action@v25...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>

Author: dependabot[bot]

.github/workflows/nix-linux.yml

@@ -15,7 +15,7 @@ jobs:
       - uses: actions/checkout@v4
         with:
           fetch-depth: 0
-      - uses: cachix/install-nix-action@v25
+      - uses: cachix/install-nix-action@v26
         with:
           extra_nix_config: |
             experimental-features = nix-command flakes

.github/workflows/nix-macos.yml

@@ -22,7 +22,7 @@ jobs:
       - uses: actions/checkout@v4
         with:
           fetch-depth: 0
-      - uses: cachix/install-nix-action@v25
+      - uses: cachix/install-nix-action@v26
         with:
           extra_nix_config: |
             experimental-features = nix-command flakes