Support UEFS and other major refactoring

• Added support for passing -Engine uefs:<uefs tag>. If you use uefs, you can now run multiple builds for the same Unreal Engine version in parallel on the same build machine, as each instance is mapped to a unique path.
• The scripts now map the working directory and engine to stable paths to ensure incremental builds work correctly under CI/CD.
• Fixed IS_RUNNING_INSIDE_BUILD_GRAPH detection when performing multiple invocations in the same process.
• Removed the -MapDriveForShorterPaths flag; it's no longer necessary as paths are always mapped (see above).
• Job bootstrapping is now performed by Bootstrap_CIJob.ps1, rather than dynamically generating PowerShell scripting inside Internal_Convert_BuildGraphToBuildPipeline.ps1. This makes the bootstrapping logic much more maintainable and cross-platform (rather than generating significantly different shell code for macOS and Windows).
• Internal_RunUAT.ps1 now automatically removes broken Automation ScriptModules (handles an issue when using an installed engine build with a specific restricted platform).
• Refactored Patch_BuildGraph.ps1 to make it work more reliably.