Add a github action step to run the tests against current master

.github/workflows/test.yml

@@ -23,6 +23,9 @@ jobs:
         signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
         authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
     - run: nix-build -A checks.$(nix-instantiate --eval -E '(builtins.currentSystem)')
+    - run: |
+      NIX_BUILD=$(nix-build -A defaultPackage.$(nix-instantiate --eval -E '(builtins.currentSystem)'))/bin/nix-build
+      $NIX_BUILD --option experimental-features 'flakes' --impure --expr 'builtins.getFlake (builtins.toPath ./.).lib.testAgainst.x86_64-linux (builtins.getFlake (builtins.toPath ./.)).defaultPackage.x86_64-linux'
   check_cachix:
     name: Cachix secret present for installer tests
     runs-on: ubuntu-latest

flake.nix

@@ -504,6 +504,12 @@
           } "touch $out";
       });
 
+      lib = {
+        testAgainst = forAllSystems (system:
+          testNixVersions nixpkgsFor.${system} nixpkgsFor.${system}.nix
+        );
+      };
+
       packages = forAllSystems (system: {
         inherit (nixpkgsFor.${system}) nix;
       } // (nixpkgs.lib.optionalAttrs (builtins.elem system linux64BitSystems) {