Update Rust/Clang download link.

[waywardmonkeys]

The previous location has been deprecated and both the pre-built binaries and build-your-own directions are in the same place.

[waywardmonkeys]

With this, I ended up needing to use cargo +esp build rather than cargo +nightly build ... so I guess this needs further updates.

[ivmarkov]

Thanks for the fix - merged now.
As for needing to use cargo +esp build... yeah, I'll update it to state that you need to do rustup default esp before proceeding.

[ivmarkov]

Fixed.