Verify Ubuntu/GCC build and supercell test run

[dwfncar]

Confirm the gfortran/conda build works end-to-end on Ubuntu hardware and the supercell case produces correct output.

• Follow conda environment setup (gcc, gfortran, openmpi, netcdf, pnetcdf via conda-forge)
• Build PIO and MUSICA from source into ~/software
• eval "$(scripts/check_build_env.sh --export)" detects gfortran toolchain
• Build atmosphere core with MUSICA (make -j8 gfortran CORE=atmosphere MUSICA=true ...)
• Run supercell test case with 8 MPI ranks
• Confirm chemistry tracers (NO, NO2, O3) evolve correctly