_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>

buildstream/_yamlcache.py

@@ -207,7 +207,7 @@ class YamlCache():
             filepath = os.path.relpath(full_path, project.directory)
         else:
             filepath = full_path
-        return full_path
+        return filepath
 
     # _calculate_key():
     #