Bugfix: Make `copy` command honor `--no-lock` for source repository

The `copy` command previously did not respect the `--no-lock` option for the
source repository, causing failures with read-only storage backends. This has
now been fixed such that the option is now respected.

https://github.com/restic/restic/issues/3518
https://github.com/restic/restic/pull/3589
