"check" command incorrectly requires --source flag

[rycoll]

The rosey check command fails, with a message about requiring a source directory to process. However, it also fails when the --source flag is used.

Workaround: the command seems to work if a truthy ROSEY_SOURCE environment variable is passed in, so ROSEY_SOURCE="anything" rosey check works.

Solution: remove/modify the code which always checks for a source directory, for commands which don't expect this to be configured.