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.
These statements are taken from the `lib/test/release.nix` tests,
which previously also worked on darwin.
Unblocks https://github.com/NixOS/nix/pull/8569 when backported
