Replace nproc in macOS setup script (#11186)

Summary:
nproc may not be found on macOS causing boost not to install. Instead, switch to use existing NPROC to determine the number of logical CPU that nproc would have returned.

Pull Request resolved: #11186

Reviewed By: kgpai

Differential Revision: D64000667

Pulled By: xiaoxmeng

fbshipit-source-id: 8238f49785508b9aee10794db4eb15f1f3a167d2

scripts/setup-macos.sh

@@ -104,7 +104,7 @@ function install_boost {
   (
     cd ${DEPENDENCY_DIR}/boost
     ./bootstrap.sh --prefix=${INSTALL_PREFIX}
-    ${SUDO} ./b2 "-j$(nproc)" -d0 install threading=multi --without-python
+    ${SUDO} ./b2 "-j${NPROC}" -d0 install threading=multi --without-python
   )
 }