tests/flake: check flake outputs for all systems

Now all `supportedSystems` from flake.nix are checked instead of only
the native system of the `nix flake check` process.

.cirrus.yml

@@ -34,7 +34,7 @@ task:
 
     - name: flake
       build_script:
-        - nix flake check
+        - nix flake check --all-systems
         - ./test/nixos-search/ci-test.sh
 
     - name: shellcheck

test/run-tests.sh

@@ -263,7 +263,7 @@ isNixVersionGreaterEqual_2_15() {
 }
 
 flake() {
-    nix flake check "$scriptDir/.."
+    nix flake check --all-systems "$scriptDir/.."
 }
 
 # Test generating module documentation for search.nixos.org