Bugfix: Correctly handle `backup --stdin-filename` with directories

In restic 0.18.0, the `backup` command failed if a filename that includes
a least a directory was passed to `--stdin-filename`. For example,
`--stdin-filename /foo/bar` resulted in the following error:

```
Fatal: unable to save snapshot: open /foo: no such file or directory
```

This has been fixed now.

https://github.com/restic/restic/issues/5324
https://github.com/restic/restic/pull/5356