internal: add lock for pre-build

[lijunchen]

No description provided.

[peter-jerry-ye-code-review]

Based on the provided git diff output, here are three potential issues or suggestions for improvement in the code:

1. Unused Import: The import statement use moonutil::common::{FileLock, MoonbuildOpt}; introduces FileLock, but it is not used anywhere in the code snippet provided. This can lead to unnecessary clutter and potential confusion. It's advisable to remove unused imports to keep the code clean and maintainable.

2. Potential Race Condition: The code acquires a lock using let _lock = FileLock::lock(&common)?; before accessing the database. While this is a good practice to prevent race conditions, it's crucial to ensure that the lock is held for the shortest possible time to avoid blocking other processes. Ensure that any operations that could potentially take a long time are minimized while the lock is held.

3. Error Handling: The function load_moon_generate returns a Result type, indicating that it might fail. However, the error handling within the function is minimal. Specifically, the line let db = n2::db::open(&n2_db_path, &mut graph, &mut hashed).unwrap(); unwraps the result of n2::db::open, which will cause the program to panic if the operation fails. It's better to handle the error more gracefully, perhaps by returning it or logging an error message before propagating it.

These suggestions aim to improve code clarity, safety, and maintainability.