Improve pico uf2 release artifacts

[petermm]

pici-build.yaml(release) doesn't seem to use the full potential of https://github.com/pguyot/uf2tool yet - namely:

1. joining the two files to a single uf2 for easier download & install.
2. making "universal" uf2 to further avoid confusion and improve ease of use.

Will be used on future install page with esp32 web flasher, and this kind of instructions for the pico boards (demo at https://petermm.github.io/atomvm_flasher/)