Build Broken on Latest Nightly cause Rust Core reverted the FromStr implementation for PathBuf

[zmanian]

This rust-lang/rust#50401 broke the KMS build.

@tarcieri any thoughts on what to do?

Change the types of the arguments from PathBufs to Strings?

[tarcieri]

Sure, that's fine.

(Also: can't wait to be on stable. Should be the next release, I hope!)