[cmake] Only set the native build marker once
Summary: As it turns out, the second time it is set override the cache, which can create strange side effects.
Modify the cmake module in some trivial way.
Build with ninja, ensure that gen_context is not a problem anymore after regeneration.
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D5020