don't rely on CARGO environment variables at runtime

[passivedragon]

Seems to me like any attempt at building a release build, i.e. anything cargo independent, necessarily fails due to unwrapping an environment variable that probably isn't set.

[passivedragon]

There still exist a couple other issues as it comes to pathing, I don't think the current approach with canonicalize is working as intended.

For predictability purposes I'd suggest using the binary path as a basis for resolving relative links, probably better than forcing the current working directory, that would limit dependent projects quite a bit. If that kind of behavior is wanted, users of the library can always amend the paths passed to bevy_fmod that way.

[passivedragon]

Any thoughts on this? I'd be ready to clean this up, if the general idea seems fine.

[Salzian]

Hey, I'm currently not actively developing this library. I'll try to take a look at this next weekend.

[Salzian]

Hey, we made some changes to the way we load banks and plugins. Can you check if your PR is still relevant? Thanks!