OvmfPkg: PlatformCI: Add PATH_TO_OS Flag

This adds the ability to pass PATH_TO_OS to the
PlatformBuild.py scripts in the PlatformCI folder.

This allows easy booting to an OS (or generically adding a
drive) while using the standard PlatformBuild.py for OvmfPkg.

Signed-off-by: Oliver Smith-Denny <osde@microsoft.com>