Canonical is typically hiring for many positions all the time, what I would recommend you do is follow the trends of what Canonical is releasing.
What jobs are available now
First, you should visit the Canonical jobs offering page and see what positions (and the qualifications) they have.
The best way to learn about different stacks being used by Ubuntu, the Ubuntu Phone, and other Ubuntu products is to start working on them! You can contribute to the project without being a Canonical employee which might be a great way to learn a new technology, language, or stack. While I can't speak to if this actually helps candidates get hired or not I imagine it doesn't hurt to be acquainted with the project the company works on that you aspire to work for.
Canonical is a growing company, so there's generally going to be a need for new people in different arenas of the industry. Keep your eyes open for jobs you're interested in but the goal should be "this interests me I want to learn it" not so much "I must learn this to work here"