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

When passing the `--no-lock` flag to the copy command, the source directory still attempted to issue a lock, causing failures on read-only source directories. `--no-lock` is now respected and copy no longer attempts a lock.

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