Ensure android NDK is installed regardless of runner.

This fixes the workflow on runner images where the NDK is not
installed. It bundles together a few other changes to ensure
CMake is executed correctly and make it easier to diff CMake
log in the GHA UI with the content in swift-toolchain.yml

* Always run gha-setup-vsdevenv even if runners.os != 'Windows'
  * It provides us with the path to CMake
* Switch to nttdl/setup-ndk to install the Android NDK.
* Always specify Android-specific MAKE defines, but enable others
  iff matrix.os == Android.
  * CMAKE_SYSTEM_NAME=${{ matrix.os }} controls which system we're
    building for, so there's no need to use matrix.extra_flags for
    Android-specific defines. Inlining them ensures powershell
    variable expansion works, and makes it easier to copy paste
    CMake commands for for local reproduction.