Allow disablement of platform detection when the platform and version details are provided explictly

Today oryx runs the detector for each supported platform regardless of whether we have passed `--platform` and `--platform-version` values during build. For most scenarios, we might want to disable running of all detectors and just rely on user provided values to do the env setup and build. Running of all detectors results in fetching/installing of all detected platforms even if they are not required for the build. Oryx already has config flags which allow disablement of select platforms(eg. DISABLE_NODEJS_BUILD). It might be nice to have another config like `DISABLE_PLATFORM_DETECTION` to skip the detection cycle completely. The `oryx prep` command also has a `--skip-detection` flag but there is no such equivalent for `oryx build` command.