Pin rust-toolchain to 2020-07-11

[Centril]

• Adds rust-toolchain with 2020-07-11 as the last known good nightly while Regression in nightly-2020-07-22: collection encountered polymorphic constant rust-lang/rust#74614 is pending a fix.

• Updates script/init.sh to query rust-toolchain and install the toolchain + wasm32 for it.

• Removes rustup update calls per review suggestion.

• Removes +nightly from wasm-gc call per review suggestion.

[maxsam4]

@Centril Changes look good to me. Some additional suggestions about the script: Since we are now pinning the toolchain via the toolchain file, we can remove the rustup update commands from the init script. Also, when installing wasm-gc, we don't need to specify +nightly.

[Centril]

@maxsam4 Addressed. :)