Reinstate some hotspot variant build jobs in the regular tag-triggered runs

[sxa]

We seem to have lost the jobs in the pipeline that verified the hotspot variant builds.
Looks at the triggered builds it seems to have a default BUILD_CONFIG_URL of https://raw.githubusercontent.com/adoptium/ci-jenkins-pipelines/master/pipelines/jobs/configurations which I guess is a prefix, but https://github.com/adoptium/ci-jenkins-pipelines/blob/master/pipelines/jobs/configurations/jdk21u.groovy in there seems to still have the hotspot Linux/aarch64 build but the triggered builds such as https://ci.adoptium.net/job/build-scripts/job/openjdk21-pipeline/267/parameters doesn't seem to have it.

We should resolve that and reinstate the validation of the hotspot variant builds.