Disable idb stdout buffering for logs

Summary:
Changelog: Disable idb stdout buffering for logs. Show logs on iOS devices without buffering.

Currently, idb buffers log output if it's redirected to a non-TTY. It is the default behavior for Python (read more about it [here](https://eklitzke.org/stdout-buffering)).
It leads to users seeing logs only in large chunks because the size of the buffer is substantial.
Setting PYTHONUNBUFFERED to a non-empty string disables Python output buffering.

Reviewed By: lblasa

Differential Revision: D32984711

fbshipit-source-id: 3e0a889b6b60b6d266f4a0c1894db937868c7433

desktop/flipper-server-core/src/devices/ios/IOSBridge.tsx

@@ -70,7 +70,11 @@ export function idbStartLogListener(
   return child_process.spawn(
     idbPath,
     ['log', '--udid', udid, '--', ...getLogExtraArgs(deviceType)],
-    {},
+    {
+      env: {
+        PYTHONUNBUFFERED: '1',
+      },
+    },
   );
 }