fn toolchain_url() references nonexistent nightly toolchain releases

[EricArcherman]

For aarch64 linux machines:

fn toolchain_url() @

jolt/jolt-core/src/host/toolchain.rs

Line 160 in 137552a

fn toolchain_url() -> String {

target_lexicon::HOST is not limited to the three released .tar's (https://github.com/a16z/rust/releases/tag/nightly-8af9d45d5e09a04832cc9b2e1df993fd1ce49d02).

To fix: add more images like aarch64-unknown-linux-gnu in addition to aarch64-apple-darwin, x86_64-apple-darwin, x86_64-unknown-linux-gnu.

Opened same issue on a16z/rust - a16z/rust#15.