ci: Retry WP core downloads in test-plugin-update

[tbradsha]

Closes MONOREP-197

Similar to #45779, sometimes WP core downloads fail as well, e.g.:
https://github.com/Automattic/jetpack/actions/runs/19178820882/job/54830260653

For now let's just do a single retry after a variable sleep. A more robust solution might be coming in WP-CLI:

• Add automatic retry mechanism for HTTP requests wp-cli/wp-cli#6140
• Add Support for Retry in WP CLI Core Download Command wp-cli/core-command#258

Proposed changes:

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

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

Testing instructions:

The general effect can be tested like so:
wp core download asdf.asdf || sleep $(( 0 + RANDOM % 8 )) && wp core download fdsa.fdsa

[github-actions]

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[anomiex]

Looks reasonable to me

[anomiex]

Oops. We both missed that this should have been something like

[ -f /var/www/html/xmlrpc.php ] || wp core download || { sleep $(( 30 + RANDOM % 8 )) && wp core download; }

[tbradsha]

#45831