add flake module

flake/nix-unit.nix

@@ -0,0 +1,34 @@
+{
+  this,
+  flake,
+  deps,
+  ...
+}: let
+  inherit
+    (deps)
+    nixpkgs
+    nix-unit
+    ;
+
+  inherit
+    (this.util)
+    forAllSystems
+    ;
+in {
+  checks = forAllSystems (system: {
+    default =
+      nixpkgs.legacyPackages.${system}.runCommand "tests"
+      {
+        nativeBuildInputs = [nix-unit.packages.${system}.default];
+      }
+      ''
+        export HOME="$(realpath .)"
+        # The nix derivation must be able to find all used inputs in the nix-store because it cannot download it during buildTime.
+        nix-unit --eval-store "$HOME" \
+          --extra-experimental-features flakes \
+          --override-input nixpkgs ${nixpkgs} \
+          --flake ${flake}#tests
+        touch $out
+      '';
+  });
+}