EdkRepo: Prevent environment variables from causing install failures

If the following two environment variables in the global system scope:

PIP_INDEX_URL
PIP_TARGET

It will breaks the EdkRepo installer's ability to place files into the
Python site-packages directory. To workaround this, the installer should
temporarily delete those environment variables.

Cc: Ashley E Desimone <ashley.e.desimone@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Puja Pandya <puja.pandya@intel.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Prince Agyeman <prince.agyeman@intel.com>
Cc: Erik Bjorge <erik.c.bjorge@intel.com>
Signed-off-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
Reviewed-by: Ashley DeSimone <ashley.e.desimone@intel.com>
Reviewed-by: Erik Bjorge <erik.c.bjorge@intel.com>

edkrepo_installer/EdkRepoInstaller/InstallWorker.cs

@@ -576,6 +576,8 @@ public void PerformInstall(Action<bool, bool> ReportComplete, Action<int> Report
             Action ReportFailure = new Action(delegate () { FailureReported = true; });
             Environment.SetEnvironmentVariable("PYTHONHOME", null);
             Environment.SetEnvironmentVariable("PYTHONPATH", null);
+            Environment.SetEnvironmentVariable("PIP_INDEX_URL", null);
+            Environment.SetEnvironmentVariable("PIP_TARGET", null);
             if (VendorCustomizer.Instance != null)
             {
                 VendorCustomizer.Instance.WriteToInstallLog = new Action<string>(InstallLogger.Log);