Fix DSC_RESOURCE_PATH env var to also limit exe search

[SteveL-MSFT]

PR Summary

The original intent of DSC_RESOURCE_PATH was to constrain searching not only manifests but also executables to the paths of that env var. However, the current code only used it for finding resource manifests and relied on the existing PATH to find executables.

The change is to set the process PATH env var to same as DSC_RESOURCE_PATH such that the original intent is respected. We should make this clear in the docs. If the user wants to temporarily add to PATH using this env var, they should append PATH to it like:

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

PR Context

Fix #814

[SteveL-MSFT]

Looks like a bunch of tests relied on the previous behavior and I'll need to fix them