Sanitize JDKs on the AIX machines

We currently have a mix of JDKs on the AIX machines - and some default to a J9 VM instead of Temurin (Either 7 or 8), some have no default, and this can lead to inconsistent behavior.

The build machines generally seem to have a Temurin 8 available and defined by the `JDK7_BOOT_DIR` variable in the machines' jenkins definitions. We should ensure that this is consistent across all of our AIX machines. We could also use this opportunity to start using "well-known" locations for the boot JDKs - specifically locations that are searched by default by the openjdk build process that would not require the variables to be set.

Identified in https://github.com/adoptium/infrastructure/pull/2288#issuecomment-2263315262 where an ineffective `keytool` was being picked up when scheduling a JDK8 build on a test box.

Locate the Jenkins definitions for all AIX machines and compare how JDK7_BOOT_DIR and the available Temurin JDKs are configured. Read the openjdk build process locations referenced in the issue, then verify that JDK 7/8 builds select the intended Temurin JDK and effective keytool consistently on every AIX machine.

Written by the indexing model from the issue text.