Add BUILD_VERSION to lib.rs

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.

fish-rust/build.rs

@@ -14,7 +14,7 @@ fn main() {
         .file("fish-rust/src/compat.c")
         .compile("libcompat.a");
 
-    let rust_dir = std::env::var("CARGO_MANIFEST_DIR").expect("Env var CARGO_MANIFEST_DIR missing");
+    let rust_dir = env!("CARGO_MANIFEST_DIR");
     let target_dir =
         std::env::var("FISH_RUST_TARGET_DIR").unwrap_or(format!("{}/{}", rust_dir, "target/"));
     let cpp_fish_src_dir = format!("{}/{}", rust_dir, "src/");