Import get_flag_value() from WP_CLI\Utils before using.

[barryhughes]

As identified in the linked issue, we are attempting to use a namespaced function without importing or fully qualifying it, leading to a fatal error when attempting to create an action via WP CLI.

Closes #1270.

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Steps to replicate

• Using the current stable release (3.9.2), try running wp action-scheduler create test now and you will experience a fatal error.
• Switch to this branch and repeat the test. The action should successfully be created.

Changelog

Fixed fatal error occuring when creating an action via WP CLI.

[barryhughes]

✍🏼 Probably worth noting, I don't see the same problem in any of the related classes: we either import the function, or fully qualify it. That said, I do see a warning when using wp action-scheduler action get <id> which I'll create a separate PR for.

[barryhughes]

Unit tests are failing in many cases, but it seems to be because of this part of the setup process:

curl -L https://github.com/woocommerce/phpunit/archive/add-compatibility-with-php8-to-phpunit-7.zip -o /tmp/phpunit-7.5-fork.zip

woocommerce/phpunit seems to have gone away, or perhaps permissions have changed?

[albarin]

✅ Confirmed the fatal error is no longer happening after this fix.

[barryhughes]

✍🏼 Just a note that I lack the powers to merge this change (though perhaps we want to wait on #1273 being addressed so we can verify this doesn't break anything, which of course is totally fine).