Fix macOS conda-forge OpenMP builds: install clang_osx-* activation packages

[Copilot]

macOS conda-forge CI jobs fail with clang: error: unsupported option '-fopenmp' (and, for the flexiblas job, 'omp.h' file not found). Root cause: conda-forge's "minimally activated compilers" change means the compilers metapackage now depends on the bare clang package, which provides only a versioned compiler binary and never sets CC/CFLAGS/CPPFLAGS/LDFLAGS on conda activate. With no unversioned clang on PATH inside the conda env and CC unset, the build tools fall back to the system Apple clang, which doesn't support -fopenmp and can't find conda-installed llvm-openmp headers.

Changes

• continuous_integration/install.sh: when INSTALL_LIBOMP=conda-forge on Darwin, additionally install clang_osx-<arch>/clangxx_osx-<arch> (arch detected via uname -m). These packages ship the activation scripts that set CC/CXX/CFLAGS/CPPFLAGS/LDFLAGS pointing at $CONDA_PREFIX, restoring the pre-"minimal activation" behavior.
• .github/workflows/test.yml: set INSTALL_LIBOMP: "conda-forge" on the pylatest_flexiblas macOS job so it goes through the same fixed activation path (it hit the same root cause via a missing omp.h).
• Debugging: print which clang, clang --version, and CC/CPPFLAGS/LDFLAGS right after conda activate on macOS, to make it obvious in CI logs which compiler is actually picked up.