Bump cachix/install-nix-action from v12 to v13

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from v12 to v13.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](https://github.com/cachix/install-nix-action/compare/v12...8d6d5e949675fbadb765c6b1a975047fa5f09b27)

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/pr.yml

@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
-    - uses: cachix/install-nix-action@v12
+    - uses: cachix/install-nix-action@v13
       with:
         nix_path: nixpkgs=channel:nixos-unstable
     - run: ./ci/test.sh

.github/workflows/update.yml

@@ -14,7 +14,7 @@ jobs:
     - uses: actions/checkout@v2
       with:
         fetch-depth: '0'
-    - uses: cachix/install-nix-action@v12
+    - uses: cachix/install-nix-action@v13
       with:
         nix_path: nixpkgs=channel:nixos-unstable
     - name: update nur / nur-combined
@@ -26,7 +26,7 @@ jobs:
     needs: update_nur
     steps:
     - uses: actions/checkout@v2
-    - uses: cachix/install-nix-action@v12
+    - uses: cachix/install-nix-action@v13
       with:
         nix_path: nixpkgs=channel:nixos-unstable
     - name: update nur-search/data/packages.json