`DSC_RESOURCE_PATH` should be prefixed to `PATH`

[SteveL-MSFT]

Summary of the new feature / enhancement

Currently, users can set env var DSC_RESOURCE_PATH to override searching PATH for resource manifests. However, the executable found in the resource manifest still relies on PATH to find the exe.

Proposed technical implementation details (optional)

Seems like we should just prefix DSC_RESOURCE_PATH to PATH when it's defined so that resource manifest is still constrained to the user defined env var and searching for the exe prefers the user defined location.