This implements `app_version` on Linux by using an optional,
compile-time `RELEASE_VERSION` env var that can be set.
We settled on the `RELEASE_VERSION` as the name, since it's similar to
`RELEASE_CHANNEL` which we use in Zed.
cc @ConradIrwin @mikayla-maki
Release Notes:
- N/A
Co-authored-by: Bennet <bennetbo@gmx.de>