Modify firmware build to use single thread

Change build process to use a single thread.

Author: reflander

.github/workflows/Build_LEDE_OP_x86.yml

@@ -84,8 +84,8 @@ jobs:
     - name: Build firmware
       working-directory: ./openwrt
       run: |
-        echo -e "$(nproc) thread build."
-        make -j$(nproc) V=s
+        echo -e "1 thread build."
+        make -j1 V=s
 
     - name : Upload artifact
       uses: actions/upload-artifact@master