wasm builder fails when running with a specific toolchain

[jbg]

rustup target add wasm32-unknown-unknown --toolchain nightly-2019-08-22
cargo +nightly-2019-08-22 build

fails with

error: failed to run custom build command for `polkadot-runtime v0.6.0 (polkadot/runtime)`

Caused by:
  process didn't exit successfully: `polkadot/target/debug/build/polkadot-runtime-8e31e940c712ba37/build-script-build` (exit code: 1)
--- stderr
   Compiling wasm-build-runner-impl v1.0.0 (polkadot/target/debug/build/polkadot-runtime-eec245fcb90ce820/out/wasm_build_runner)
    Finished dev [unoptimized + debuginfo] target(s) in 0.90s
     Running `polkadot/target/debug/build/polkadot-runtime-eec245fcb90ce820/out/wasm_build_runner/target/debug/wasm-build-runner-impl`
Rust WASM toolchain not installed, please install it!

because https://github.com/paritytech/substrate/blob/master/core/utils/wasm-builder/src/lib.rs#L154 hardcodes rustup run nightly cargo rather than using the CARGO environment variable to find cargo from the toolchain that’s in use.

[bkchr]

I had used CARGO in the beginning, but it would break for some use cases. Looks like I need to think again about it.

[emkman]

I am unable to install the latest subkey because of this issue on OS X