test(scanner): fix Windows flake in the quick-scan artist image spec (#6093)

The spec asserted on artistID("Kraftwerk") and intermittently found zero artists
on Windows. The scan did import the artist; it was then made invisible.

RefreshStats selects touched artists with a strict artist.updated_at >
library.last_scan_at (persistence/artist_repository.go:466). Windows' wall clock
has ~15ms granularity, so a new artist written by a quick scan can land in the
same tick as the previous scan's last_scan_at and be excluded. Its
library_artist.stats then stays at the '{}' default and the unscoped cleanup
DELETE removes the row, after which selectArtist's INNER JOIN on library_artist
hides the artist from GetAll.

Backdate last_scan_at before the scan so the comparison is unambiguous, matching
the fix already applied to the search_normalized spec below it.

scanner/scanner_test.go

@@ -387,6 +387,11 @@ var _ = Describe("Scanner", Ordered, func() {
 			files := fsys.MapFS
 			files["Kraftwerk/Autobahn/01 - Autobahn.mp3"] = kraftwerk(track(1, "Autobahn"))
 			fsys.SetFiles(files)
+			// Backdate the previous scan so this one's new artists are unambiguously newer:
+			// RefreshStats picks touched artists with a strict artist.updated_at >
+			// library.last_scan_at, and Windows' coarse clock can put both in one tick.
+			_, err := db.Db().ExecContext(ctx, "UPDATE library SET last_scan_at = ?", time.Now().Add(-time.Hour))
+			Expect(err).ToNot(HaveOccurred())
 			Expect(runScanner(ctx, false)).To(Succeed())
 			resolveQueuedArtwork()