Update CMake to specify FXC.EXE to use

[walbourn]

Updates the CompileShaders batch to use LegacyShaderCompiler environment variable if defined, and updates the CMake to set it based on find_program logic.

Includes fix for -Wsign-conversion warning with clang.

Updated YAML pipeline triggers to include changes to shader command script. Also includes GitHub Actions pipeline updates for some additional build scenarios.