Allow updater to check for updates after downloading one (#31066)

This PR brings back https://github.com/zed-industries/zed/pull/30969 and
adds some initial testing.

https://github.com/zed-industries/zed/pull/30969 did indeed allow Zed to
continue doing downloads after downloading one, but it introduced a bug
where Zed would download a new binary every time it polled, even if the
version was the same as the running instance.

This code could use a refactor to allow more / better testing, but this
is a start.

Release Notes:

- N/A

crates/activity_indicator/src/activity_indicator.rs

@@ -485,7 +485,7 @@ impl ActivityIndicator {
                         this.dismiss_error_message(&DismissErrorMessage, window, cx)
                     })),
                 }),
-                AutoUpdateStatus::Updated { binary_path } => Some(Content {
+                AutoUpdateStatus::Updated { binary_path, .. } => Some(Content {
                     icon: None,
                     message: "Click to restart and update Zed".to_string(),
                     on_click: Some(Arc::new({