linux: Use optional compile-time RELEASE_VERSION variable (#11490)

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>