Bugfix: fix restic dump of tar file with 32-bit binary

In restic up to 0.14.0, the restic dump from a 32-bit binary of a
snapshot of standard input that was created in Windows has as a
result a tar file whose content has a negative uid and gid. As a
result, gnu tar exits with failure status whenever it tries to
access such a tar file. With this fix, the tar file that is now
dumped from a 32-bit binary has content with non-negative uid and
gid.

https://github.com/restic/restic/issues/4103
https://github.com/restic/restic/pull/4104