Unable to install any packages/plugins in current bundle

🐛 Bug

Unable to install any packages/plugins in current bundle.

To Reproduce

Steps to reproduce the behavior:

1. Download recent bundle build: https://github.com/napari/napari/actions/runs/988280586
2. Try to install any plugins or other packages in plugins > Install/Remove plugins…
3. Get the error: error: unrecognized arguments: --upgrade

Latest version of pip has not actually removed --upgrade so it’s unclear to me what is going on. For reference the code that is failing is here:

https://github.com/napari/napari/blob/3aef5a5e6ab6c4fb4a83202040f1234ee4aca90b/napari/_qt/dialogs/qt_plugin_dialog.py#L82-L93

[goanpeca]

I modified the bundled files adding a QMessageBox and ran that on the install method inside .

/Applications/napari.app/Contents/Resources/app/napari/_qt/dialogs/qt_plugin_dialog.py

    def install(self, pkg_list: Sequence[str]):
        cmd = ['-m', 'pip', 'install', '--upgrade']
        if running_as_bundled_app() and sys.platform.startswith('linux'):
            cmd += [
                '--no-warn-script-location',
                '--prefix',
                user_plugin_dir(),
            ]

        from qtpy.QtWidgets import QMessageBox
        QMessageBox.information(None, "test", " ".join([self.process.program()] + cmd + list(pkg_list)))
        self.process.setArguments(cmd + list(pkg_list))
        if self._output_widget:
            self._output_widget.clear()
        self.process.start()

[goanpeca]

Pinging @jaimergp