Use site's PHP version when running WP-CLI commands

[fluiddot]

Related to https://github.com/Automattic/studio/issues/468.

Proposed Changes

• Pass the site's PHP version when executing a WP-CLI command.

Testing Instructions

• Download the following test plugin: test-plugin.zip
• Start the app with the command npm start.
• Create a site.
• Navigate to WP-Admin.
• Navigate to the Plugins page.
• Install the test plugin.
• Activate the test plugin.
• Observe the site throws an error. This is due to the plugin using a deprecated function create_function which was removed in PHP version 8.
• Change the PHP version to 7.4 in the Settings tab of the app.
• Try to activate the test plugin again.
• Observe no errors are thrown.
• Switch to another app and back again to the Studio app. This will trigger WP-CLI commands to fetch the installed plugins and themes of the running site.
• Observe no errors are thrown in the app logs.

Pre-merge Checklist

• [x] Have you checked for TypeScript, React or other console errors?