chore: update nix isntall action (#48)

Co-authored-by: Eike <eikek@posteo.de>
Co-authored-by: Eike Kettner <eike.kettner@sdsc.ethz.ch>

Author: olevski

.github/workflows/ci.yml

@@ -3,8 +3,8 @@ name: ci
 on:
   pull_request:
   push:
-      branches:
-        - main
+    branches:
+      - main
 
 concurrency:
   group: ci-${{ github.event.pull_request.number || github.ref }}
@@ -19,7 +19,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Install nix
-        uses: cachix/install-nix-action@V27
+        uses: cachix/install-nix-action@v31
         with:
           extra_nix_config: access-tokens = github.com=${{ github.token }}
 
@@ -43,7 +43,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Install nix
-        uses: cachix/install-nix-action@V27
+        uses: cachix/install-nix-action@v31
         with:
           extra_nix_config: access-tokens = github.com=${{ github.token }}