scdaq: Install `gcc-toolset-12`

The package includes gcc 12 and gdb through the toolset, along with experimental support for C++20, C++23.

FYI, feature matrix for C++23 support: https://en.cppreference.com/w/cpp/compiler_support/23

NOTE: built on top of the ubi8.7 base image, conforming to the changes to be merged by !20 (merged).