decrease connect-timeout to 5s

For people self-hosting caches that can be occasionally down, the
default timeout is very long. This is annoying if you are trying to
update your binary cache at the same time you are trying to update
another machine. Same if cachix has one of its rare hiccups.

We tested this value of 5s in srvos now for years and we like to travel
around the world with shitty internet, so it should be still reasonable
high.

src/libstore/include/nix/store/filetransfer.hh

@@ -30,7 +30,7 @@ struct FileTransferSettings : Config
         {"binary-caches-parallel-connections"}};
 
     Setting<unsigned long> connectTimeout{
-        this, 0, "connect-timeout",
+        this, 5, "connect-timeout",
         R"(
           The timeout (in seconds) for establishing connections in the
           binary cache substituter. It corresponds to `curl`’s