Add 'download-buffer-size' setting

We are piping curl downloads into `unpackTarfileToSink()`, but the
latter is typically slower than the former if you're on a fast
connection. So the download could appear unnecessarily slow. (There is
even a risk that if the Git import is *really* slow for whatever
reason, the TCP connection could time out.)

So let's make the download buffer bigger by default - 64 MiB is big
enough for the Nixpkgs tarball. Perhaps in the future, we could have
an unlimited buffer that spills data to disk beyond a certain
threshold, but that's probably overkill.

src/libstore/filetransfer.hh

@@ -47,6 +47,12 @@ struct FileTransferSettings : Config
 
     Setting<unsigned int> tries{this, 5, "download-attempts",
         "How often Nix will attempt to download a file before giving up."};
+
+    Setting<size_t> downloadBufferSize{this, 64 * 1024 * 1024, "download-buffer-size",
+        R"(
+          The size of Nix's internal download buffer during `curl` transfers. If data is
+          not processed quickly enough to exceed the size of this buffer, downloads may stall.
+        )"};
 };
 
 extern FileTransferSettings fileTransferSettings;