fix: build error on `docs.rs`

src/core_ffi/rust_ffi/build.rs

@@ -7,7 +7,10 @@
 
 use cxx_build::CFG;
 
+// TODO: assert version info
+
 fn main() {
+    if std::env::var("DOCS_RS").is_err() {
         cmake::Config::new("klotski")
             .define("CARGO_BUILD:BOOL", "ON")
             .define("KLSK_ENABLE_TESTING:BOOL", "OFF")
@@ -26,6 +29,7 @@ fn main() {
             .flag("-fno-rtti")
             .flag("-fno-exceptions")
             .compile("klotski");
+    }
 
     println!("cargo:rerun-if-changed=src/bridge.rs");
     println!("cargo:rerun-if-changed=adapter/short_code.cc");