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; [
+  nativeBuildInputs = [
-    nix
+    nixVersions.stable
+  ] ++ (with pkgs; [
     jq
     bc
-  ];
+  ]);
 } ''
   # Needed to make Nix evaluation work
   export TEST_ROOT=$(pwd)/test-tmp

lib/tests/nix-for-tests.nix

@@ -0,0 +1,17 @@
+{ pkgs
+}:
+
+# 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 from the Nix that runs
+# CI.  Bonus: the tests build way faster.
+#
+# See also: https://github.com/NixOS/nix/issues/7582
+
+builtins.mapAttrs (_: pkg:
+  if builtins.isAttrs pkg
+  then pkg.override { withAWS = false; }
+  else pkg)
+  pkgs.nixVersions

lib/tests/release.nix

@@ -1,8 +1,9 @@
 { # The pkgs used for dependencies for the testing itself
   # Don't test properties of pkgs.lib, but rather the lib in the parent directory
   pkgs ? import ../.. {} // { lib = throw "pkgs.lib accessed, but the lib tests should use nixpkgs' lib path directly!"; },
-  nix ? pkgs.nix,
+  nix ? pkgs-nixVersions.stable,
-  nixVersions ? [ pkgs.nixVersions.minimum nix pkgs.nixVersions.unstable ],
+  nixVersions ? [ pkgs-nixVersions.minimum nix pkgs-nixVersions.unstable ],
+  pkgs-nixVersions ? import ./nix-for-tests.nix { inherit pkgs; },
 }:
 
 let