Do not cache version in cmake build

Summary: The cmake build produce stale version numbers as a result of the caching.

Test Plan:
Build with cmake.

Increment the version number of bitcoin-abc in /CMakeLists.txt .

Rebuild, make sure the version actually changed in the generated version.h

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4228