lib.tests: build nix without flaky aws-sdk-cpp

The aws-sdk-cpp tests are flaky.

Since pull requests to staging cause nix to be rebuilt, this means
that staging PRs end up getting false CI failures due to whatever is
flaky in the AWS SDK tests.  Since none of our CI needs to (or
should be able to) contact AWS S3, let's just omit it all.  Bonus:
the tests build way faster.

lib/path/tests/default.nix

@@ -6,16 +6,19 @@
     overlays = [];
     inherit system;
   },
+  nixVersions ? import ../../tests/nix-for-tests.nix { inherit pkgs; },
   libpath ? ../..,
   # Random seed
   seed ? null,
 }:
+
 pkgs.runCommand "lib-path-tests" {
-  nativeBuildInputs = with pkgs; [
-    nix
+  nativeBuildInputs = [
+    nixVersions.stable
+  ] ++ (with pkgs; [
     jq
     bc
-  ];
+  ]);
 } ''
   # Needed to make Nix evaluation work
   export TEST_ROOT=$(pwd)/test-tmp