message: fix: build.rs failed without git dir
  Ignore the error when failed to read .git/HEAD.
  Closes
diff: @@ -38,7 +38,7 @@ fn main() {
  println!("cargo:rerun-if-changed=build.rs");
  println!("cargo:rerun-if-changed=.git/HEAD");
  - let head = std::fs::read_to_string(".git/HEAD").expect("read .git/HEAD");
  + let head = std::fs::read_to_string(".git/HEAD").unwrap_or_default();
  if head.starts_with("ref: ") {
  println!("cargo:rerun-if-changed=.git/{}", head[5..].trim());
  }
  