Fix builds using pico.toochain with SDK 1.5.0

[Daft-Freak]

The blit SDK itself is not affected by this as it includes the pico-sdk import in the root CMakeLists. Anything using the toolchain file (like the boilerplate) is a bit broken.

Prevents errors like:

[cmake] CMake Error at /home/daftfreak/workspace/pico/pico-sdk/tools/CMakeLists.txt:61 (find_package):
[cmake]   find_package called with invalid argument ".."
[cmake] Call Stack (most recent call first):
[cmake]   /home/daftfreak/workspace/pico/pico-sdk/src/rp2_common.cmake:47 (pico_add_uf2_output)
[cmake]   32blit-pico/CMakeLists.txt:140 (pico_add_extra_outputs)
[cmake]   32blit-pico/CMakeLists.txt:148 (blit_executable_int_flash)

[Gadgetoid]

Ooof! Did your autobuildamajig catch these?

[Daft-Freak]

Sort-of. I tagged a release on rocks-and-diamonds to fix an issue the auto-builder was having... and the PicoSystem action failed