PyOxidizer now provides MSI installers and pre-built Linux binaries.
So we install that way. This significantly reduces the time to bootstrap
a new machine in automation, as building PyOxidizer from source on a low
core count machine takes several minutes.
Because `pyoxidizer.exe` is no longer in PATH (this may be a bug in its
MSI installers), we adjust the Windows powershell snippets used in
automation to ensure its installed path comes first in PATH.
This change temporarily breaks the ability of the automated environment
to use the in-repo pyoxidizer.bzl configuration file, as there are
backwards-incompatible changes with the upgrade that need to be
reflected. We'll handle those in subsequent commits.