Fetcher: re-use URLSession (#976)

Otherwise we start to periodically get RST's from GitHub, possibly
because of too many connection opens, which has an effect of cancelling
previously received bytes.

These RST's can be observed in tcpdump/Wireshark or Console, emitted
from the libusrtcp.dylib library, com.apple.network subsystem, for the
Tart process:

>tcp_input [C59.1.1.1:3] flags=[R] seq=1805021659, ack=0, win=0 state=CLOSED rcv_nxt=1805021659, snd_una=1752355607

You can also observe the "Received Bytes" in "Activity Monitor" for
the Tart process while pulling ghcr.io/cirruslabs/macos-runner:sequoia,
and this value will periodically decrease.

Sources/tart/Fetcher.swift

@@ -1,6 +1,6 @@
 import Foundation
 
-fileprivate var urlSessionConfiguration: URLSessionConfiguration {
+fileprivate var urlSession: URLSession = {
   let config = URLSessionConfiguration.default
 
   // Harbor expects a CSRF token to be present if the HTTP client
@@ -13,14 +13,15 @@ fileprivate var urlSessionConfiguration: URLSessionConfiguration {
   // [2]: https://github.com/cirruslabs/tart/issues/295
   config.httpShouldSetCookies = false
 
-  return config
-}
+  return URLSession(configuration: config)
+}()
 
 class Fetcher {
   static func fetch(_ request: URLRequest, viaFile: Bool = false, progress: Progress? = nil) async throws -> (AsyncThrowingStream<Data, Error>, HTTPURLResponse) {
+    let task = urlSession.dataTask(with: request)
+
     let delegate = Delegate()
-    let session = URLSession(configuration: urlSessionConfiguration, delegate: delegate, delegateQueue: nil)
-    let task = session.dataTask(with: request)
+    task.delegate = delegate
 
     let stream = AsyncThrowingStream<Data, Error> { continuation in
       delegate.streamContinuation = continuation