Instead of putting target/ and in the source
directory, we can tell our cargo script to put both target
and our cargo-home in meson's build directory. This means
that when triggering a rebuild through meson via tooling
like Builder, Fractal will actually rebuild.
Previously we were using the env! macro to determine
build-time variables like version, app ID, and locale dir.
Instead of relying on env vars, we can create a configuration
file with meson and import it.
I've added the env var CARGO_HOME pointing to target directory to make
it cacheable when you build Fractal with builder to speed up building.
I've also added an env var check to difference between DEBUG mode and
RELEASE mode so you can configure GnomeBuilder to build the debug
version.