Fix LTO regressions in nightly toolchain
Created by: ascjones
Fix for https://github.com/paritytech/ink/issues/391.
Recent nightly has broken since https://github.com/rust-lang/rust/pull/71925 (see https://github.com/rust-osdev/cargo-xbuild/issues/69#issuecomment-627231724)
- Upgrades version of
cargo-xbuild
which adds LTO flag to sysroot build: https://github.com/rust-osdev/cargo-xbuild/pull/71. - Disables lto flag when generating metadata which fixes
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. rel: #40