Add conda lib to static linker search path for builds (meta-pytorch#1972)

Summary:
Pull Request resolved: meta-pytorch#1972

Our install instructions say to use `conda install libunwind`, but the rust compiler
is unable to locate it.
Add a `-L` flag when building from within python to add the conda environment libraries.
This should help rust locate the libraries it needs.

Reviewed By: colin2328

Differential Revision: D87665225

fbshipit-source-id: 75393719c1feec435aa31915687929ce74f1cd43

Author: dulinriley

setup.py

@@ -154,6 +154,10 @@ def run(self):
         "link-arg=-Wl,-rpath,$ORIGIN/../../..",
         "-C",
         "link-arg=-Wl,-rpath," + conda_lib,
+        # Add the conda lib to the search path for the linker, as libraries like
+        # libunwind may be installed there.
+        "-L",
+        conda_lib,
     ]
     if py_lib:
         flags += ["-C", "link-arg=-Wl,-rpath," + py_lib]