_yamlcache.py: Correct bug in `YamlCache._get_filepath()`

The correct file path was computed but never returned.  This was
never caught because of a test missing consumption of one of its
parameters.  This commit resolves the bug mentioned in #722 leaving
the test to be amended by a future commit.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>