ci: pass correct args to the act test

.github/workflows/test-per-system.yml

@@ -13,7 +13,8 @@ on:
         type: string
       oldest_installer_version:
         description: 'Oldest supported Nix installer version to test (e.g., nix-2.8.0)'
-        required: true
+        required: false
+        default: 'nix-2.8.0'
         type: string
 
 env:
@@ -94,4 +95,4 @@ jobs:
       with:
         nix_path: ${{ env.nixpkgs_channel }}
         install_url: https://releases.nixos.org/nix/${{ inputs.oldest_installer_version }}/install
-    - run: nix-build tests/test-build.nix
+    - run: nix-build tests/test-build.nix

.github/workflows/test.yml

@@ -65,4 +65,5 @@ jobs:
         ./bin/act push \
           -P ubuntu-latest=ghcr.io/catthehacker/ubuntu:js-24.04 \
           -j simple-build \
-          --matrix os:ubuntu-latest
+          --matrix runs-on:ubuntu-latest \
+          --matrix system:x86_64-linux