support read rustup toolchain from env

[Bandeapart1964]

Hi, I added a line to read rustup toolchain version from shell environment “RUSTUP_TOOLCHAIN”, it allows user to use an installed or speficied toolchain, rather than force use 'stable'.

ps. The environment name from the rustup book

thx your work!

[heckmon]

Okay, but what's the point of using another version rather than the stable?

[Bandeapart1964]

stable is not a fixed version number, if it build fails for some reasons, then I can specified to last available version.

another reason is when i already installed a toolchain and use it in my project, I don't expect download another one when building code forge.

[Bandeapart1964]

since the cargokit use rustup specified toolchain before run cargo, so it will ignore default toolchain in system, that means code forege always use stable rust toolchain, even I set another one as default in my system