rust nightly build fail on mac osx, succss after downgrade

[yrong]

• version

➜  darwinia git:(master) git log
 commit b2d34b576d87442155b0986fa9502a5a01be58c1 (HEAD -> master, tag: v0.7.0-alpha.2, origin/master, origin/HEAD)
Author: Xavier Lau <[email protected]>
Date:   Sat Sep 19 19:19:30 2020 +0800

    Release 0.7.0-alpha.2 (Dev) (#540)
...

• toolchain

 ➜  darwinia git:(master) sw_vers
ProductName:	Mac OS X
ProductVersion:	10.15.6
BuildVersion:	19G2021
 ➜  darwinia git:(master) rustc --version
rustc 1.48.0-nightly (fb1dc34a8 2020-09-21)
➜  darwinia git:(master) cargo --version
cargo 1.48.0-nightly (8777a6b1e 2020-09-15)

• build error

 ➜  darwinia git:(master) cargo build --release
error: failed to run custom build command for `darwinia-runtime v0.7.0-alpha.2 (/Users/ronyang/Dev/myrust/darwinia/runtime/darwinia)`

Caused by:
  process didn't exit successfully: `/Users/ronyang/Dev/myrust/darwinia/target/debug/build/darwinia-runtime-f8e49155a19b2dc2/build-script-build` (exit code: 1)
...
      Finished dev [unoptimized + debuginfo] target(s) in 3m 11s
       Running `/Users/ronyang/Dev/myrust/darwinia/target/debug/wbuild-runner/darwinia-runtime13453144019960859995/target/x86_64-apple-darwin/debug/wasm-build-runner-impl`
  Rust WASM toolchain not installed, please install it!

build success following this link:

https://stackoverflow.com/questions/63993569/how-to-resovle-rust-wasm-toolchain-not-installed-please-install-it

[aurexav]

rust-lang/rust#76698