Build - Add plugins/backup to previous vendor copy logic

[jwebbdev]

Changes proposed in this Pull Request:

• Adds plugins/backup to have vendor files copied to avoid conflicts when upgrading from 1.0.1 to 1.1.0

Jetpack product discussion

Internal references:

p9dueE-4fc-p2
p1642671449497200-slack-CBG1CP4EN

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

• Click on the Build check below, and in the summary view click to download "jetpack-build"
• Uncompress the downloaded archive, navigate to the Jetpack Backup plugin (jetpack-backup-plugin), and check that the files can be found at the old location (vendor/automattic/...)
• Zip just the Jetpack Backup plugin (make sure the folder is named just jetpack-backup before zipping).
• Create a new JN site without any plugins
• SSH into the site, and tail the debug.log.
• Go to Plugins > Add New > Search for Jetpack Backup and install from .org (v1.0.1) - Activate
• Connect your site to WordPress.com from Jetpack > Backup
• Go to Plugins > Add New > Upload plugin, and upload the zip you've just created.
• It should offer you to update from Jetpack Backup 1.0.1 to Jetpack Backup 1.1.1-alpha (if you don't see the "this plugin is already installed..." prompt then a step was likely missed).
• Run the update; you should see no issues on the site or in the log.

[github-actions]

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[jeherve]

This tests well for me. 🚢

[jeherve]

@jwebbdev If you want this to ship with the Backup 1.1.0 release, you'll want to cherry pick dace448 into the backup/branch-1.1.0 branch, so it can be automatically ported to the same branch in the mirror repo.

[sdixon194]

Cherry picked to backup release branch in 372a611 and cleaned up in f6a15e6