Always use original location instead of shadow-copied location when loading RProvider plugins

This may or may not help fix #122 . I've used this snippet many times before (e.g., in the facio test runner) to solve a similar issue. I wasn't able to build/run the RProvider tests on my machine -- the FAKE build failed with some odd errors -- so someone will need to check whether this PR actually fixes the issue as expected.
This should be fine here, because the argument uses `EscapedCodeBase`