In CMake this used a `version` file in the CARGO_MANIFEST_DIR, but
relying on that is problematic due to change-detection, as if we add
`cargo-rerun-if-changed:version`, cargo would rerun every time if the file does
not exist, since cargo would expect the file to be generated by the
build-script. We could generate it, but that relies on the output of `git
describe`, whose dependencies we can only limit to anything in the
`.git`-folder, again causing unnecessary build-script runs.
Instead, this reads the `FISH_BUILD_VERSION`-env-variable at compile time
instead of the `version`-file, and falls back to calling git-describe through
the `git_version`-proc-macro. We thus do not need to deal with extraneous
build-script running.
- This allows running `cargo fmt/clippy/test/etc` from root
- Ideally the root should be the fish-rust package instead of being virtual, but
that requires changed to CMake/Corrosion. This change should instead be
completely compatible with our existing setup.
- This also means we will only have on `Cargo.lock` for all current and future
crates.
