Use CARGO_MANIFEST_DIR for add example build script

Author: LegNeato

examples/cuda/cpu/add/build.rs

@@ -1,8 +1,10 @@
 use cuda_builder::CudaBuilder;
 
 fn main() {
-    CudaBuilder::new("../../gpu/add_gpu")
-        .copy_to("../../resources/add.ptx")
+    let manifest_dir = std::env::var("CARGO_MANIFEST_DIR").expect("CARGO_MANIFEST_DIR exists");
+    let manifest_dir = std::path::Path::new(&manifest_dir);
+    CudaBuilder::new(manifest_dir.join("../../gpu/add_gpu"))
+        .copy_to(manifest_dir.join("../../resources/add.ptx"))
         .build()
         .unwrap();
 }