generateAntoraYml remains up-to-date when its attributes have changed

[wilkinsona]

For example, the changes in a405c42 added two new attributes (url-log4j2-docs and url-quartz-javadoc) but generateAntoraYml may not run as a result of such changes as it doesn't correctly consider the attributes as an input. This can cause the docs to fail to build due to missing attributes.