jetpack-mu-wpcom: fix undefined is_plugin_active fatal on wpcom

[samiff]

Proposed changes:

Fix the following fatal on wpcom that surfaced from #28932

PHP Fatal error:  Uncaught Error: Call to undefined function Automattic\Jetpack\is_plugin_active() in ...jetpack-mu-wpcom-plugin/production/vendor/automattic/jetpack-mu-wpcom/src/class-jetpack-mu-wpcom.php:52

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

Internal: p1677262859532259/1677260163.446909-slack-CBG1CP4EN

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

No.

Testing instructions:

• Test the newly built mu-wpcom-plugin on your sandbox and make sure there is no fatal:

bin/jetpack-downloader test jetpack-mu-wpcom-plugin fix/mu-wpcom-is-plugin-active

[github-actions]

Are you an Automattician? You can now test your Pull Request on WordPress.com. On your sandbox, run

bin/jetpack-downloader test jetpack-mu-wpcom-plugin fix/mu-wpcom-is-plugin-active

to get started. More details: p9dueE-5Nn-p2

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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[anomiex]

Seems sensible.