Upgrade via a ZIP file doesn't work if password isn't cached

[figureone]

This is a follow-up to #9757.

Aloha, we noticed that upgrading via a ZIP file will not work if the server prompts for an FTP password via the Connection Information screen (e.g., if FTP_PASS is not defined in wp-config.php).

Note that this only applies if direct file I/O is not used for updates (e.g., FS_METHOD is ssh2, ftpext, or ftpsockets).

The flow is:

1. User uploads a plugin or theme ZIP file via Appearance > Themes > Upload Theme or Plugins > Installed Plugins > Add New.
2. Connection Information screen appears, asking for Hostname, FTP Username, FTP Password, and Connection Type (see screenshot: ​https://imgur.com/a/xl64OeL).
3. After providing credentials, the new "This theme/plugin is already installed" screen appears, comparing Current to Uploaded (see screenshot: ​https://imgur.com/a/cnntWm9).
4. If you click "Replace current with uploaded," you will return to the Connection Information screen. After providing credentials again, you loop back to step 3, and are stuck in the cycle.

I've also attached a short screencast illustrating the loop (on a local vagrant set up on Ubuntu 20.01 LTS using vsftpd to perform file I/O, using FS_METHOD = ftpext).