Zeb Evans is stretching before he goes on stage in a rainy, windswept Dublin. Evans is messing about rather than seriously limbering up for a grilling in front of an audience of hundreds of people on the main stage at the tech conference SaaStock.  The founder of $4 billion valued productivity start-up ClickUp is famed for his colourful shirts combined with white trousers, and he doesn’t disappoint. I’ve gone with my dull uniform of a dark jumper and navy jeans (best not to clash.)  Evans is the founder and chief executive of ClickUp. Founded in 2017 in San Diego, ClickUp was…