Building twice fails when re-fetching ffmpeg

[kornelski]

I've got a build error due to missing deps, and when I ran cargo build for a second time I got this:

fatal: destination path 'ffmpeg-3.4' already exists and is not an empty directory.
thread 'main' panicked at 'called Result::unwrap() on an Err value: Error { repr: Custom(Custom { kind: Other, error: StringError("fetch failed") }) }', src/libcore/result.rs:906:4

I presume there's an unconditional git clone involved. Deleting the directory before the clone (or skipping clone when it exists) would prevent the problem.

cargo clean works as a workaround, but it's slower than re-running cached build would be.