Fix failed population of `PICO_EXTRAS_PATH` when fetched from git

[ithinuel]

When running cmake with

cmake -B build -DPICO_SDK_FETCH_FROM_GIT=y -DPICO_EXTRAS_FETCH_FROM_GIT=y

configuration fails because PICO_EXTRAS_PATH incorrectly points to build rather than build/_deps/pico_extras-src.

[ithinuel]

gentle ping @lurch for a review

[lurch]

Thanks for the nudge, but I'm not directly working on Pico-related stuff any more, so it's not up to me to review or merge this PR.

[ithinuel]

Thank you for your reply. Based on the git history and the profiles' contribution, I guess the next canditate is @kilograham ?