Consolidate the usage of conan profiles (such as cppstd value).

[Adnn]

Currently, the value of cppstd is present in the profile, but is overriden (to the same value) by the main_build.yml workflow.
Maybe, the profile could be a single source of truth for the build settings, and the workflow should use them from the profile.

Also, we would imagine the conan profiles becoming a build-matrix dimension (potentially handling Debug vs Release, several cppstd versions, several compilers, ...)

Also, currently we define the C and CXX env vars, and let the default profile be generated with the compiler version automatically extracted from there. Should the compiler version become part of the host profile?

• It would give a natural place to change compiler versions, by committing to conan-config
• Drawbacks:
  • the profile should become the single source of truth for compiler version too
  • there would be no more "tolerance" on the actual compiler used to build (currently, the profile is generated with the currently active compiler, so it is very tolerant in this regard).