feat: use CARGO_ENCODED_RUSTFLAGS instead of RUSTFLAGS

[ashutoshvarma]

Summary

Use CARGO_ENCODED_RUSTFLAGS instead of RUSTFLAGS in contract-build crate. This does not affect/break anything.

Why?

I wanted to use contract-build inside cargo build script for compiling ink! contract fixtures for testing, here is a example build script - https://github.com/AstarNetwork/Astar/blob/e148ddf271577b4e36662afe4b9ec820c235a384/tests/xcm-simulator/build.rs. But with the current use of RUSTFLAGS it's not possible.

Since with Rust 1.55.0, RUSTFLAGS is not set for build scripts - rust-lang/cargo#10111. On top of that when cross-compiling (in our case wasm32-unknown-unknown), cargo does not apply RUSTFLAGS for build scripts by design. That means RUSTFLAGS will totally be ignored in build script environment, that includes setting env from inside build script.

Solution

Simply use CARGO_ENCODED_RUSTFLAGS instead - rust-lang/cargo#10111 (comment)

[cla-bot-2021]

User @ashutoshvarma, please sign the CLA here.

[ascjones]

LGTM. Thanks!