I would need about 90m of cable. I know that the maximum of UTP is 100m, so it would be feasible with it
Are you sure about this? It's unlikely you'll be able to run this cable directly... you'll add some distance going around corners and bends here and there. If you don't get this right, some very weird things can happen. I recommend placing a small switch at the near end of the outdoor gap in one or both buildings to act as a bridge, and thus ensure you're not anywhere close to your distance limit.
I also want to address how you run the wire between the buildings. You cannot just string copper wire through the air, as you see done with telephone or power lines. Over time you can build up a voltage differential from one side of the link to the other, even with simple/short building to building links. I've heard this can even build to level of potential electrocution, and while that might be urban legend I know you can burn out equipment this way. Now you could just do the same kinds of things the phone and power companies to do to handle this issue, but I really prefer to just have the cable buried. Last time I had this done I was quoted $14 per foot to have a underground conduit bored, though that pricing can vary wildly depending on where you are and what you have to deal with. I've seen it as cheap as $8 per foot and as high as $26, but I don't doubt it could get much higher yet. Still, I think this is by far the way to go. If it's all grass and private property, you might also be able to just trench it, but 20m is at the edge of where you're likely better off with a boring machine.
There is some cheap wifi bridging equipment out there now. A wifi bridge will be easier to set up outdoors, but it's less ideal in terms of performance, ability, and security. At your distance I would expect it to be reliable enough, but you will be subject to rf interference. Still, even the low-end stuff is gonna cost at least $250 by the time you consider radios and power for both ends. Depending on where you are, that might be enough to make the cost of a boring your conduit seem reasonable, and again, I think this should be your first choice.