[CAS] Bypass IO sandbox in `OnDiskGraphDB::store()` (#202760)
The sandbox is currently only bypassed in
`OnDiskGraphDB::createStandaloneLeaf()`, but the calling function
`store()` may also initiate some IO by calling `MappedTempFile::keep()`
whose parent `TempFile::keep()` calls `sys::fs::closeFile()`. Let's move
the sandbox disablement to the caller to avoid violations.
rdar://177274700