Forward PYTHONPATH to XHarness iOS script

[premun]

When iOS simulators slow down / freeze up, we detect this, retry the work item and reboot the machine.
Since the iOS payload script is run through launchctl to run in a user session with GUI rendering, we lose some of the env vars on the way so we have to pass them down.

This PR re-initializes the PYTHONPATH variable.

Resolves: dotnet/runtime#45181

Example problem this solves:

+ /usr/local/bin/python3 -c 'from helix.workitemutil import request_infra_retry; request_infra_retry('\''Retrying because iOS Simulator application install hung'\'')'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ModuleNotFoundError: No module named 'helix'
+ /usr/local/bin/python3 -c 'from helix.workitemutil import request_reboot; request_reboot('\''Rebooting because iOS Simulator application install hung '\'')'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ModuleNotFoundError: No module named 'helix'

https://helixre107v0xdeko0k025g8.blob.core.windows.net/dotnet-runtime-refs-pull-45839-merge-13b48d1686ba4343b7/Microsoft.Bcl.AsyncInterfaces.Tests/console.fa2f6cc5.log?sv=2019-07-07&se=2020-12-29T16%3A46%3A22Z&sr=c&sp=rl&sig=tn4cgwt0brU45SKlzBlHEbsrbDNIhpZNsqrQkidt7Ss%3D

[MattGal]

Probably won't have to do this to make similar scripts for Windows / Linux as all processes are children of the client there.