build(deps): 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] <[email protected]>

.github/workflows/nix.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4
-      - uses: cachix/install-nix-action@v25
+      - uses: cachix/install-nix-action@v26
       - uses: cachix/cachix-action@v14
         with:
           name: numtide
@@ -35,7 +35,7 @@ jobs:
         with:
           # Nix Flakes doesn't work on shallow clones
           fetch-depth: 0
-      - uses: cachix/install-nix-action@v25
+      - uses: cachix/install-nix-action@v26
       - uses: cachix/cachix-action@v14
         with:
           name: numtide
@@ -51,7 +51,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4
-      - uses: cachix/install-nix-action@v25
+      - uses: cachix/install-nix-action@v26
       - uses: cachix/cachix-action@v14
         with:
           name: numtide