Use gcc8 for legacy build
Up to now the legacy build was using gcc-10 because it was available in the store.
It is not satisfying because, for example, some headers of c++20 are already available in gcc-10 but are not in previous version. As we use gcc-8 elsewhere we have to ensure the build is ok with gcc8.
This MR adds the download and install of gcc8 for legacy build.
The CI crashes for legacy build. It is expected because some headers have been introduced recently and are not available with gcc8. A fix will come right after this MR.