Summary:
We started getting the message
```stderr: eden/fs/utils/SpawnedProcess.cpp:798:21: error: 'getIOExecutor' is deprecated: getIOExecutor is deprecated. To use the global mutable executor use getUnsafeMutableGlobalIOExecutor. For a better solution use getGlobalIOExecutor. [-Werror,-Wdeprecated-declarations]
```
I don't see why we would need a mutable executor here so I chose `getGlobalIOExecutor` over `getUnsafeMutableGlobalIOExecutor`.
Reviewed By: kmancini
Differential Revision: D27912276
fbshipit-source-id: 95b1053f72c2b4eb2746e3c40c0cf76b69d90d6e