Darker github action ignores working-directory

[dshemetov]

Describe the bug
Darker github action ignores working-directory setting.

To Reproduce
Steps to reproduce the behavior: take a look at these two commits in my demo repo, one and two.

Expected behavior
Darker CI action should use the working-directory setting.

Additional context
Vaguely related to my previous report here #522. Thanks for your help!

[akaihola]

Thanks @dshemetov, I'll definitely take a look at this!