Fix CPack NSIS homepage

Summary: BITCOINABC_HOMEPAGE_URL does not exist. Use the existing homepage that is already set.

Test Plan:
Setup win64 cross build:

cd depends
make build-win64
cd ..
mkdir build-win64
cmake -GNinja .. -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Win64.cmake -DBUILD_BITCOIN_SEEDER=OFF

Build the package and inspect build contents:

ninja package
less _CPack_Packages/x86_64-w64-mingw32/NSIS/project.nsi  # verify URL is set
less CPackConfig.cmake  # verify homepage is set

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5372