HTTPClient: disable URLSession cache (#2668)

Turns out this was breaking an HTTP 304 integration test in #2659. I'm
very confused why we hadn't seen it before.

Possibly it didn't matter for normal requests, but it was breaking
signature verification because the `nonce` was different on the new
request, and therefore it wouldn't match the cached signature.

Sources/Networking/HTTPClient/HTTPClient.swift

@@ -42,6 +42,7 @@ class HTTPClient {
         config.httpMaximumConnectionsPerHost = 1
         config.timeoutIntervalForRequest = requestTimeout
         config.timeoutIntervalForResource = requestTimeout
+        config.urlCache = nil // We implement our own caching with `ETagManager`.
         self.session = URLSession(configuration: config,
                                   delegate: RedirectLoggerSessionDelegate(),
                                   delegateQueue: nil)