build: Run chmod instead of using cmake's file CHMOD command
* cmake/modules/G10AddDistTargets.cmake, cmake/modules/G10AddReleaseTargets.cmake, cmake/modules/G10GitHooks.cmake: Replace file(CHMOD ...) with execute_process(COMMAND chmod +x ...).
The Docker image we use for building the AppImage has cmake 3.17, but
file CHMOD was added in 3.19.
- GnuPG-bug-id: T7262