[copybara-service]

ci: Vary one dimension at a time from a single origin

Each build workflow previously defined its own matrix, repeating compiler,
C++ standard, build-mode, and Python-version combinations across operating
systems and build systems. Even without the Intel macOS workflows, the two
remaining macOS workflows spawn 6 jobs per run against GitHub Actions'
5-concurrent-macOS-job limit, and the macOS and Windows Bazel workflows
build with -c opt, compiling most files twice (PIC and non-PIC).

Restructure the 6 build workflows around a single origin (ubuntu-26.04,
clang-22, Bazel 9.x, fastbuild, C++17, Python 3.12) in
amd64_linux_bazel and vary one dimension at a time from it, using a flat
matrix: include: list across all workflows:

• Compiler, C++ standard, Python version, and Bazel version variations run
  only on Linux, where runners are fast and plentiful, and CMake on Linux
  varies only CMake-specific settings (version, generator, build type).
• macOS drops from 6 jobs to 2 (1 fastbuild job in arm64_macos_bazel
  and 1 Xcode multi-config job in arm64_macos_cmake, exercising the
  APPLE target properties and Xcode generator), and Windows Bazel
  switches from opt to fastbuild (retaining windows-2022 and -c dbg
  for MSVC _DEBUG).
• arm64_macos_cmake and amd64_windows_cmake are kept as single jobs
  even though their OSes and CMake are already tested separately, because
  our CMake config has APPLE, WIN32 (BUILD_SHARED_LIBS=ON DLL copy),
  and multi-config (Xcode / Visual Studio LIBRARY_OUTPUT_DIRECTORY)
  logic that Linux CMake and Bazel do not exercise.

[Mizux]

please keep it to test XCode and Apple Clang