wip: try to install cachix flake via ubuntu-latest

.forgejo/workflows/check.yml

@@ -3,15 +3,16 @@ on: [push]
 
 jobs:
   Check:
-    runs-on: nix-flakes
+    runs-on: ubuntu-latest
     steps:
       - name: Check out repository code
         uses: https://code.forgejo.org/actions/checkout@v4
+      - name: Install nix
+        uses: https://github.com/cachix/install-nix-action@v23
       - name: Prepare cachix
         uses: https://github.com/cachix/cachix-action@v12
         with:
           name: pub-solar
-          installCommand: nix shell nixpkgs#cachix
       - name: Run flake checks
         run: |
           nix --print-build-logs --verbose --accept-flake-config flake check