Revert "ci: bump install nix action version"

This reverts commit a69f3e5.

Author: smoothprogrammer

.github/workflows/main.yaml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Setup Nix
-        uses: cachix/install-nix-action@v30
+        uses: cachix/install-nix-action@v26
       - name: Checkout
         uses: actions/checkout@v4
       - name: Cache Nix