Add read-only cache for prebuilts back

see https://github.com/cachix/cachix-action#read-only-cache

Author: SuperSandro2000

.github/workflows/prebuilt-25.05.yml

@@ -19,4 +19,8 @@ jobs:
       with:
         name: microvm
         authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+    - uses: cachix/cachix-action@v16
+      if: github.ref != 'refs/heads/main'
+      with:
+        name: microvm
     - run: nix build -L .#prebuilt --override-input nixpkgs github:nixos/nixpkgs/release-25.05

.github/workflows/prebuilt-unstable.yml

@@ -19,4 +19,8 @@ jobs:
       with:
         name: microvm
         authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+    - uses: cachix/cachix-action@v16
+      if: github.ref != 'refs/heads/main'
+      with:
+        name: microvm
     - run: nix build -L .#prebuilt --override-input nixpkgs github:nixos/nixpkgs/nixos-unstable