Bugfix: Use `--cache-dir` argument for `check` command

`check` command now uses a temporary sub-directory of the specified directory
if set using the `--cache-dir` argument. If not set, the cache directory is
created in the default temporary directory as before.
In either case a temporary cache is used to ensure the actual repository is
checked (rather than a local copy).

The `--cache-dir` argument was not used by the `check` command, instead a
cache directory was created in the temporary directory.

https://github.com/restic/restic/issues/1880
