put flake input overrides into a helper function

lib/flake-inputs.nix

@@ -0,0 +1,16 @@
+{ ... }:
+{
+  /**
+    Generate nix-unit input overrides for tests
+
+    # Example
+    ```nix
+    inputOverrides = clanLib.flake-inputs.getOverrides inputs;
+    ```
+  */
+  getOverrides =
+    inputs:
+    builtins.concatStringsSep " " (
+      builtins.map (input: " --override-input ${input} ${inputs.${input}}") (builtins.attrNames inputs)
+    );
+}