Run sync when issue is labeled with Sync Backport Changelog

[noisysocks]

Stacked onto #63792.

This PR goes a bit further and updates the Sync Core Backport workflow to run when an issue is labeled with the 🤖 Sync Backport Changelog label.

You can see it working here:

https://github.com/noisysocks/gutenberg/actions/runs/10034996226/job/27730360753

[kevin940726]

LGTM 🚢

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[noisysocks]

Thanks Kai! I'll merge tomorrow morning then create a new sync issue for 6.7 and we'll see if it works.

[ellatrix]

We can skip checking out the repo to check for changes if it's triggered by the label

[noisysocks]

We have to check out the repo because of the awk in the script below. We can skip the git diff though. I've done that.

[noisysocks]

I've updated the script to skip the git diff when possible and I've gone back to using env.changes as it turns out this bug was just to do with a typo. See #63792 (comment).

Tested everything in my Gutenberg fork.

Here's merging a PR that has no changes in backport-changelog:

https://github.com/noisysocks/gutenberg/actions/runs/10050944095/job/27779706306

Here's merging a PR that has changes in backport-changelog:

https://github.com/noisysocks/gutenberg/actions/runs/10050926085/job/27779663400

Here's adding the label to an issue:

https://github.com/noisysocks/gutenberg/actions/runs/10050917093/job/27779640416

[noisysocks]

Yay it works #63827