(#18931) Improve warning messages on windows

Previous to this change a windows agent could
see a deprecation warning even if the manifests
explicitly specified ignore if, say, pluginsync
tripped the copy_source_values logic.

Also previous to this change, a manifest could
specify source_permissions => use_when_creating
for Windows agents, although that makes no more sense
than source_permissions => use for Windows agent.

So this change:
* only issues a warning if the manifest explicity
  specifies source_permissions => use or
  source_permissions = use_when_creating, since
  those settings aren't desirable on windows
* disallows use_when_creating on windows agents