Fix newer notes incorrectly being skipped when importing successive exports

[iamllama]

Related? https://forums.ankiweb.net/t/update-notes-if-newer-only-works-one-time/52064

Fixes the "skipped notes when importing" issue in #3589

Let's say someone exports a deck X as an apkg (1), modifies it, exports it again (2), modifies it, exports it again (3)
And on a new profile: imports apkg 1, imports apkg 2, and tries to import apkg 3

Currently, when existing notes are updated while importing an apkg (2), the mtime is set to the time of the import.

This means that the notes in apkg 3 are now considered to be older than existing notes, and are skipped when trying to import it.

This is incorrect behaviour, since the user didn't manually modify any notes after each import.

With this change, the incoming note's mtime is preserved to allow importing successive exports

[dae]

@RumovZ any thoughts? I can't recall if the mtime bump was something deliberate and required, or just a byproduct of the existing update_note() implementation

@iamllama changes look good on my end, and thanks for tackling that clippy todo while you were there!