Grab pre-build picotool

[thejpster]

Installing picotool from the unstable repo takes 90 seconds. We could grab it from https://github.com/raspberrypi/pico-sdk-tools/releases instead, which would take about 5 seconds.

[jannic]

Using a packaged version of picotool has the advantage that dependencies are automatically resolved so we don't end up with missing shared libraries. One caveat here is that the package is not built for Ubuntu but for Debian, so there might still be some compatibility issues. Once picotool is available for Ubuntu, we should surely change that.

However, 90 seconds indeed is quite a lot. I was able to reduce the installation time to about 30s with #962. What do you think, is that acceptable?

[thejpster]

30 is better than 90 so that’s a good start. But I think our users are likely to use the pre-built picotool so I like the idea of finding issues with it before they do.