Pico SDK does not correctly include time.h causing compilation failures in the toolkit.

[sgstreet]

Until Pico-SDK Issue #1781 is resolved please use a patched version development branch found at https://github.com/sgstreet/pico-skd

[sgstreet]

Backing up the picolibc version to 1.8.6 also works around the problem. Issues have be raised with both the pico-sdk and picolibc teams.