FASTBuild: Allow files in different subdirs to be in the same unity bucket

The FASTBuild generator puts source files from different subdirectories
into different `FastbuildObjectListNode` instances. This ensures that
files with the same name will not write to the same object file. A unity
bucket can only contain files from a single `FastbuildObjectListNode`
instance, so a side effect of this logic is that unity buckets can only
contain files in the same subdirectory. This change fixes the problem by
skipping the subdirectory check logic for files with unity enabled.

Fixes: #27457