Enhancement: backup no longer updates file access times on Linux

When reading files during backup, restic caused the operating system to update
the file access times. Note that this does not apply to filesystems with
disabled file access times.

Restic now instructs the operation system not to update the file access time,
if the user running restic is the file owner or has root permissions.

https://github.com/restic/restic/pull/2816