cmake: skip boost dependency on ALIAS executable targets

cmake: skip boost dependency on ALIAS executable targets

The current add_executable override in Boost does not support alias
targets. Although Ceph currently has no alias targets that are
affected by this limitation, addressing this issue now will benefit
future developments and personal projects.
This change enhances the robustness of the override logic, ensuring
compatibility with alias targets moving forward.

Signed-off-by: Seena Fallah <seenafallah@gmail.com>