Don't use CARGO_MANIFEST_DIR in include!() path

include!() expects a path relative to the current file, so we don't need
to dynamically construct its argument at all. In fact, the current
argument breaks the build when CARGO_MANIFEST_DIR holds a relative path
from the working directory and the working directory isn't the crate
root.

build.rs

@@ -1,6 +1,6 @@
 use std::env;
 
-include!(concat!(env!("CARGO_MANIFEST_DIR"), "/src/load.rs"));
+include!("src/load.rs");
 
 fn main() -> Result<()> {
     println!("cargo:rerun-if-changed=assets/oid_db.txt");