Make Self-Host service run as Local System for nightly runs and PR's

Our nightly-build Self-Hosted services is being run as the local user, not Local System. Cross-machine testing has verified this causes problems with some authentication tests.

Proposed solution is:
1. Install PsExec.exe on the nightly build machine (and add to setup instructions for future reference)
2. Set the environment variable PSEXEC_PATH to the folder containing PsExec.exe

When this is done, the existing scripts to start the self-hosted service will run as Local System