feat: be/local/shell.Spawn should use UserDataDir for workdir not /tmp

[starpit]

1. we aren't cleaning up the ioutil.TempDir() that is used for the workdir
2. avoid /tmp for long-running processes
3. avoid /tmp because maybe minio is confused by it (search for O_DIRECT in the minio issues)

for 1, we can piggyback on killProcessGroupOnContextCancellation() and also auto-clean up when the process exits normally