Use latest ci-toolkit, 3.9.1, to print better logs when caching

[mokagio]

See changes from Automattic/a8c-ci-toolkit-buildkite-plugin#135

In chatting with @ParaskP7 and @wzieba , we thought it be useful to surface cache performance in CI using the new ci-toolkit version.

Example, the Ruby Gems cache:

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr24002-1dab555
	Version	25.6
	Bundle ID	org.wordpress.alpha
	Commit	1dab555
	App Center Build	WPiOS - One-Offs #11387

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr24002-1dab555
	Version	25.6
	Bundle ID	com.jetpack.alpha
	Commit	1dab555
	App Center Build	jetpack-installable-builds #10419

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.