store/darwin: don't try and pass in keychains to export
store/darwin: don't try and pass in keychains to export

The only filepath that's supported is where to write the plist file.

```
$ /usr/bin/security trust-settings-export
Usage: trust-settings-export  [-s] [-d] settings_file
    -s                  Export system trust settings (default is user)
    -d                  Export admin trust settings (default is user)

        Export trust settings.
```

adamdecaf/cert-manage,adamdecaf/cert-manage