`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.

[SteveL-MSFT]

While working on this, I think the intent of DSC_RESOURCE_PATH is to allow higher level tools to constrain the search path, but this should include search path for the executables and not just the resource manifests. As such, this means PATH should be overwritten with DSC_RESOURCE_PATH. However, a side effect of this is that any exe that then depends on another exe may not find it unless it's defined within DSC_RESOURCE_PATH. I think we might just need to document this and tooling that wants the prefix behavior should do that within DSC_RESOURCE_PATH. For example, in PowerShell:

$env:DSC_RESOURCE_PATH = "foo;bar;$env:PATH"