Get correct directory for writing .force_transcode file

[OllysCoding]

As discussed on Discord, this bug means that the .force_transcode is actually never properly written to:

The value returned by data.get('final_cache_path') is actually /tmp/unmanic/unmanic_file_conversion-dahwh-1705019796/It's Always Sunny in Philadelphia - S07E06 - The Storm of the Century Bluray-720p-gpdmc-1705020189.mkv.

So then on the next line, it's joining that value with .force_transcode, which gives the output /tmp/unmanic/unmanic_file_conversion-dahwh-1705019796/It's Always Sunny in Philadelphia - S07E06 - The Storm of the Century Bluray-720p-gpdmc-1705020189.mkv/.force_transcode

Now the actual file is at /tmp/unmanic/unmanic_file_conversion-dahwh-1705019796/.force_transcode, but since it's getting the final cache path of the file, not the folder the file is in, the if statement is getting false. That file it's looking for doesn't exist

This fix, suggested & tested by @yajrendrag, resolves it by getting the directory, instead of the file.