[Tooling] Use Buildkite emojis for Prototype Builds

[AliSoftware]

Why?

If we don't provide an explicit app_icon to the call to prototype_build_details_comment action, this action uses the URL of the icon returned by the App Center API as a fallback.

This works well in general, except that this URL contains a signed token in its query parameter, that is only valid for a certain amount of time. Thus this ends up making that icon URL invalid (and the icon image unreachable) after some time has passed (e.g. when you go back on older PRs, like here).

How?

To fix this, we now specify the Buildkite emoji (:wordpress: or :jetpack:) explicitly to use as app_icon.

Testing

• Check that the PR comments about WP and JP prototypes builds show the right icon for each case
• Hover over the and emojis that are embedded in those comments—both for the emoji at the start of the comment before the 📲 , and the one in front of the "App Name" row—and verify that all 4 of them use a URL like https://raw.githubusercontent.com/buildkite/emojis/main/img-buildkite-64/*.png (instead of a URL to App Center servers with a token in query)

[peril-wordpress-mobile]

	Warnings
⚠️	PR is not assigned to a milestone.

Generated by 🚫 dangerJS

[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	pr20667-c6495fa
	Version	22.3
	Bundle ID	org.wordpress.alpha
	Commit	c6495fa
	App Center Build	WPiOS - One-Offs #5693

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	pr20667-c6495fa
	Version	22.3
	Bundle ID	com.jetpack.alpha
	Commit	c6495fa
	App Center Build	jetpack-installable-builds #4723

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

[spencertransier]

This looks good to me!