Learning EVERY OS is like the Duke Of Edinburgh Awards (UK) - Either nearly everyone has one and for those that don't it doesn't affect your employ-ability chances/how your CV looks. As long as you can google how to use a windows/mac you'll be fine.
With respect to your final question - always expanding your knowledge base is a great idea - helps you make better decisions and analyse things in a different way.
I will say that, if you are in the UK, I've noticed that there is not a specific demand for a certain language/framework over another. It depends on the company and the region, for example - South Wales seems to be favouring PHP but other regions in England favour <insert language here>.
Additionally (UK), the gap between comp sci degree courses and the world is quite a gap and therefore I would suggest - keep up with the forums #HashNode medium.com/reddit /r/webdev etc. :)