Take RUSTFLAGS from .cargo/config

[jbrd]

If you're working on a project that has a .cargo/config file, and that config file happens to set RUSTFLAGS, then currently, the vscode extension will compile the project with a different set of RUSTFLAGS to the rest of your project. The workaround currently, is to set rust.rustflags in settings.json, but it would be nicer if the extension could pick up this setting from .cargo/config automatically

[tan-wei]

I agree with you.