Use system FFmpeg if binaries are not included in the package

[OpenBagTwo]

While bundling everything up into one self-contained executable makes sense for 90% of users, requiring that the package include FFmpeg in the bin folder means that FoxNap can't be distributed via standard Python package distribution channels (read: pip and pipx).

Therefore, it should be the behavior of the resource pack generator to check if FFmpeg is in the expected spot, and then, if it's not, try to use the system FFmpeg / FFprobe.