Clean up dangling temporary directories on Windows

[t-mw]

Easiest solution I can think of to avoid temporary directories building up as discussed in https://github.com/jaemk/self_update/pull/69. This creates a separate temporary directory just for the exe backup, which is manually removed on Windows the next time the updated exe is run. Could go in a patch release (last PR from me hopefully).

Backing up the exe to a separate temporary directory allows us to:

• check its contents before deleting, because it will always contain just the exe, not the download.
• have the other temporary directory for the zip download be automatically removed by the tempfile crate now that it isn't used to store the exe backup.

[t-mw]

Getting a docker rate limit error on one of the CI jobs, need to try again later.

[t-mw]

Tests passed now

[jaemk]

Nice, thanks @t-mw !