Allow the customization of sysroot_src in the generated rust-project.…

[photex]

When specifying a local rust toolchain with an unconventional rust-src location; this attribute will allow you to influence what the sysroot_src path in the resulting rust-project.json.

In my particular case I'm working in a conda environment which I configure as a new_local_repo in my project.

There I define a rust toolchain and my rustc sources live in $CONDA_PREFIX/lib/rustlib/src/rust/library. By default, if I'm unable to provide this path, the resulting sysroot_src in my project ends up resolving to the equivalent of $CONDA_PREFIX/library.

[photex]

Thank you for the review and suggested improvements @illicitonion. :D