only run Windows build on Windows runner

.github/workflows/netexec.yml

@@ -22,6 +22,10 @@ jobs:
       run: |
         pip install shiv
         python build_collector.py
+    - name: Build Windows binary
+      if: runner.os == "Windows"
+      run: |
+        pip install pyinstaller
         pyinstaller netexec.spec
     - name: Upload nxc binary
       uses: actions/upload-artifact@master