[CMAKE] Use the new FindPython module
The FindPythonInterp is replaced by FindPython since cmake 3.12.
This also solves a conflict with some pyenv installed versions (thanks @markblundeberg for reporting the issue !).
ninja all bitcoin-bench check check-security check-functional
Run the OSX Gitian build.
Reviewers: #bitcoin_abc, markblundeberg
Reviewed By: #bitcoin_abc, markblundeberg
Differential Revision: https://reviews.bitcoinabc.org/D5284