Enable string interpolation for environment shorthand

[rgwood]

Description

This enables string interpolation for the environment variable shorthand syntax.

For example, FOO=$"(123)" $env.FOO now interpolates the string and returns 123. Before this change it returned (123).

Tests

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --all --all-features -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo build; cargo test --all --all-features to check that all the tests pass

[fdncred]

thanks