[workflows] Remove custom CXXFLAGS for the abi tests (#133968)
Dropping -g1 should improve the debuginfo and hopefully make the abi
tests more accurate. I think -g1 was originally used to save disk space,
but the GitHub runners have more disk space now, so it isn't necessary.
The abi-compliance checker recommends using -Og, but it seems not to
make a difference and it's more simple just to not specify any extra
flags.