Fix Github Actions using Rust nightly on Windows/MSYS2

[lautis0503]

Two potential problems should be solved later:

1. When using rustup-init.sh to install Rust nightly, the host x86_64-pc-windows-gnu is speicified directly. Better to use variable from environment matrix mingw-w64-x86_64.
2. After Rust nightly installed, neither export new PATH nor echo PATH >>$GITHUB_ENV worked for MSYS2. The cargo command is called from its default location. Better to set the path of rust/cargo expicitly to prevent from possible change of default installation location.

The Linux and macOS workflows are untouched.
BTW, why we need to build using nightly?

[dbalsom]

Two potential problems should be solved later:

1. When using rustup-init.sh to install Rust nightly, the host x86_64-pc-windows-gnu is speicified directly. Better to use variable from environment matrix mingw-w64-x86_64.
2. After Rust nightly installed, neither export new PATH nor echo PATH >>$GITHUB_ENV worked for MSYS2. The cargo command is called from its default location. Better to set the path of rust/cargo expicitly to prevent from possible change of default installation location.

Bless you if this works. I beat my head against the wall trying it

The Linux and macOS workflows are untouched. BTW, why we need to build using nightly?

The new WASM build needs nightly for building std with atomics to enable threading with web workers.
The native build probably doesn't need it. I haven't figured out if I can set the required toolchain by target...

[dbalsom]

Thanks for fixing this!