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/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