Follow-up to #60301, which was reverted in #60321.

I removed the reqwest part of the fix because it crashed. reqwest's
`read_timeout` relies on a tokio timer, but Zed polls HTTP responses on
gpui's executor instead of a tokio runtime, so it panicked when the
response body was awaited. I'll follow up later by patching our reqwest
fork to support the poll behavior we wanted.

This fixes the download being stalled since we restart it when the
system wakes.

I also changed `App::on_system_wake` to return a `gpui::Subscription` to
be consistent with gpui's other on-system-event handlers like
`on_keyboard_layout_change` and `on_thermal_state_change`.

Release Notes:

- Fixed Zed update downloads stalling after the system wakes from sleep