allow toolchain overriding in idl build command

[skrrb]

in order to build the idl, nightly toolchain is currently hardcoded in the code

anchor/idl/src/build.rs

Line 73 in 9761ea6

const TOOLCHAIN: &str = "+nightly";

However, sometimes latest nightly may be incompatible with a given program. This pr allows to override the toolchain used by the idl build command via RUSTUP_TOOLCHAIN env, e.g.

RUSTUP_TOOLCHAIN=”nightly-2023-04-01" anchor idl build

[vercel]

@skrrb is attempting to deploy a commit to the coral-xyz Team on Vercel.

A member of the Team first needs to authorize it.

[acheroncrypto]

Thanks, this was on my todo list too.

One potential concern is that using this could cause problems with anchor build command (due to cargo build-sbf invocation), but I've checked it, and it doesn't seem to care about the RUSTUP_TOOLCHAIN env variable.

It'd also be great to support overriding from CLI args, but that can be handled in a separate PR.