fix "cargo contract build" under example directory

[anonymwy]

Within current master branch(commit hash: 268e80a), running "cargo contract build" under example directory(e.g. "examples/erc20") will cause:

error[E0658]: use of unstable library feature 'bool_to_option'
--> /Users/wuyang/src/github.com/paritytech/ink/crates/env/src/engine/on_chain/ext.rs:194:29
|
194 | (code.0 < SENTINEL).then_some(code.0)
| ^^^^^^^^^
|
= note: see issue #80967 rust-lang/rust#80967 for more information
= help: add #![feature(bool_to_option)] to the crate attributes to enable

For more information about this error, try rustc --explain E0658.
error: could not compile ink_env due to previous error

This PR fix it by following its instruction.

[anonymwy]

just found out

"warning: the feature bool_to_option has been stable since 1.62.0 and no longer requires an attribute to enable"

when trying to reproduce continuous integration error on my own machine.