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

When passing the `--no-lock` flag to the copy command, restic still attempted
to lock the source repository, causing failures on read-only storage backends.
`--no-lock` is now respected and copy then no longer creates a lock in the
source repository.

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