(#14964) Don't fail if we can't unlink the Tempfile on Windows

Previously, if the exec resource created a process, e.g. start.exe, that
executed another process asynchronously, then the grandchild would inherit
the tempfile handle, preventing puppet from being able to unlink it. This
is not an issue on POSIX systems.

This commit changes the wait_for_output method to ignore Errno::EACCES
exceptions caused when closing and unlinking the stdout tempfile. The
behavior on POSIX systems is unchanged.