fix up the builder so that this works as a nix expression

crates/spirv_backend_runner/src/lib.rs

@@ -3,6 +3,6 @@
 pub fn spirv_module() -> &'static [u8] {
     // we specifically include the bytes into this binary, for ease of packaging (v.s. attempting
     // to locate the compiled result at runtime).
-    include_bytes!("../../../target/spirv-builder/spirv-unknown-vulkan1.1/release/deps/spirv_backend.spv.dir/module")
+    include_bytes!(std::env!("SPIRV_MODULE_PATH"))
 }