Make CPack email available for all generators

Summary: This contact email will be used in the Debian generator and maybe others.

Test Plan:

ninja package
less CPackConfig.cmake  # make sure CPACK_PACKAGE_CONTACT and CPACK_NSIS_CONTACT are set properly

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5371