Add rustc's sysroot/lib to LD_LIBRARY_PATH

[naseschwarz]

The default way to use rust is rustup. Rustup maintains its own sysroot. This is fine for most rust applications, as they don't dynamically link to anything in that sysroot. rustfmt does, however, and thus must be called with the correct LD_LIBRARY_PATH.

(I'm honestly not sure whether anybody checks bootstrap.sh. Quite a few of the tests are failing and the script is not referenced anywhere.)

[ytmimi]

I don't think bootstrap.sh is used anywhere. The script looks like it was used to run rustfmt against itself.

The contributor docs already explain how to run rustfmt from source, and the recommendation is to use cargo run since it'll configure the LD_LIBRARY_PATH correctly.