On debexpo_exec, set TMPDIR to a context managed TemporaryDirectory() (Closes: #147)

In debexpo_exec(), use a TempoaryDirectory() in a context manager to ensure $TMPDIR is removed on __exit__.

This way, whether a process exit successfully, unsuccessfully or is killed, it won't leave any files in $TMPDIR.

Test added to cover:

• Successful exec
• Failed exec (Timeout)
• Unsuccessful exec with custom env

Closes: #147