Enhancement: The `backup` command no longer updates file access times on Linux

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

Restic now instructs the operating 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