build monarch_extension as extension-module (2) (#1611)

Summary:
Pull Request resolved: #1611

this is a patch of D84956118 where initialization of `rust_binding_args = ["--"] ` is changed to `rust_binding_args=[]`.

`setup-tools-rust` inserts its own `--` to pass flags to `rustc` (along with `-Zthreads=...`).

our extra `--` yields `-- -- -Zthreads=16 ...` so `-Zthreads=16` is misparsed as a positional input:
```
... --crate-type cdylib -- -Zthreads=16 ...
                   ^^^^  ^^
                 (1st) (2nd)
```

Differential Revision: D84986921

Author: shayne-fletcher

setup.py

@@ -9,6 +9,7 @@
 import shutil
 import subprocess
 import sysconfig
+import sys
 
 import torch
 
@@ -120,11 +121,8 @@ def run(self):
     readme = f.read()
 
 python_lib_dir = sysconfig.get_config_var("LIBDIR")  # path to libpython3.10.so.1.0
-
-rust_binding_args = ["--"]
-if not USE_TENSOR_ENGINE:
-    rust_binding_args.append("--no-default-features")
-rust_binding_args += [f"-C", f"link-args=-Wl,-rpath,{python_lib_dir}"]
+rpath_flag = f"-C link-args=-Wl,-rpath,{python_lib_dir}"
+os.environ["RUSTFLAGS"] = (os.environ.get("RUSTFLAGS", "") + " " + rpath_flag).strip()
 
 rust_extensions = [
     RustBin(
@@ -138,7 +136,7 @@ def run(self):
         path="monarch_extension/Cargo.toml",
         debug=False,
         features=["tensor_engine"] if USE_TENSOR_ENGINE else [],
-        args=rust_binding_args,
+        args=[] if USE_TENSOR_ENGINE else ["--no-default-features"],
     ),
 ]