health: name the skip label in the failing check's log output #455

When a health check fails, the ##[error] annotation names the problem but not the escape hatch. The whole of what a failing changelog check leaves in the log is:

##[error]The package a2ui_core needs a changelog update. Please update dart/a2ui_core/CHANGELOG.md.

The skip-changelog-check label should be mentioned to help to get exempt.