Remove some unnecessary hierarchy

I thought `default.nix` was handled specially for the purposes of automatic
parameter population but it isn't.  Instead, you just need this `args@{...}`
pattern.

.circleci/config.yml

@@ -423,7 +423,7 @@ jobs:
             cache_if_able nix-build \
               --cores 8 \
               --argstr pkgsVersion "nixpkgs-<<parameters.nixpkgs>>" \
-              nix/tests/
+              nix/tests.nix
 
   typechecks:
     docker:

nix/tests.nix

@@ -1,4 +1,4 @@
 # Build the package with the test suite enabled.
-args@{...}: (import ../../. args).override {
+args@{...}: (import ../. args).override {
   doCheck = true;
 }