mint-release-upgrade: cancelling package download does not work, may corrupt apt sources

Describe the bug
if during the release upgrade you click "cancel" while downloading packages, the upgrade flows continues instead of stopping.

To Reproduce
Steps to reproduce the behavior:

1. start release upgrade
2. click 'cancel'

Expected behavior
the upgrade flows stops and the apt sources are reverted to whatever they were before.

instead, the flowed continued, trying to download and install more stuff. i hit cancel several times, and eventually the upgrade failed (after installing some packages).

curiously, i am 99% sure that the system was running "zara" when i tried to upgrade to "zena"; but after the cancelled/failed upgrade, the apt sources were reset to "wilma"! this obviously required manual intervention to fix.

(the reason for me to cancel the upgrade was that the mirrors where reset by the upgrader, and one was slow to the point of being unusable, but this is another issue.)

Distribution:

• Linux Mint
• LMDE
• Other (please specify)

Software version:
7.1.4

Additional context
Add any other context about the problem you think might be relevant.

Start with the mint-release-upgrade release-upgrade flow and reproduce cancellation during package download. Trace how cancellation and apt-source restoration are handled; done means the upgrade stops without further installation and the sources return to their pre-upgrade state. The issue names no source files or tests, so locating the relevant entry point will require repository research.