build(deps): bump cachix/install-nix-action from 30 to 31

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 30 to 31.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](cachix/install-nix-action@v30...v31)

---
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]>

Author: dependabot[bot]

.github/workflows/test.yml

@@ -31,7 +31,7 @@ jobs:
         fetch-depth: 0
 
     # Install Nix and use that to run our tests so our environment matches exactly.
-    - uses: cachix/install-nix-action@v30
+    - uses: cachix/install-nix-action@v31
       with:
         nix_path: nixpkgs=channel:nixos-unstable
 
@@ -56,7 +56,7 @@ jobs:
         fetch-depth: 0
 
     # Install Nix and use that to run our tests so our environment matches exactly.
-    - uses: cachix/install-nix-action@v30
+    - uses: cachix/install-nix-action@v31
       with:
         nix_path: nixpkgs=channel:nixos-unstable