fix(updater): release the previous download when downloading again (#3691)

Each `Update.download()` overwrote `this.downloadedBytes` without closing the
previous resource, so the whole previous update payload stayed in the webview
resources table. The previous resource is now closed once the new download
succeeded (so a failed retry still leaves the earlier download installable).

Not covered by e2e: the fixture manifest has no valid signature, so a
download can never succeed there.

plugins/updater/guest-js/index.ts

.changes/updater-js-redownload-leak.md

@@ -0,0 +1,6 @@
+---
+updater: patch
+updater-js: patch
+---
+
+Calling `Update.download()` again now releases the bytes of the previous download instead of keeping them in memory until the webview is destroyed.