Keep the build dir exist to make sure debugger source map works correctly

[taikulawo]

Hi, I'm debugging rust app and want to step into openssl source codes in debugger, but I found source codes were removed.

openssl-src-rs/src/lib.rs

Line 147 in f08cb16

fs::remove_dir_all(&build_dir).map_err(|e| format!("build_dir: {e}"))?;

Could we keep it in OUT_DIR?