Some crates in zed use `include_str!("../../zed/RELEASE_CHANNEL")`,
which breaks when used as a dependency in a crane nix build (each crate
is built in isolation).

This adds a compile time env var to skip that check, which allows crates
in the zed repo to be used as dependencies in a crane build.

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Release Notes:

- N/A or Added/Fixed/Improved ...