chore(ci): checkout core first to cache

[mwangggg]

fixes: #315
Basically, when caching the maven files, the key that we use is unique to each version of the pom.xml file. When we checkout the Cryostat repo, which has a different pom.xml file, we are unable to hit the exact key that we saved the cache under because the hashes of the two different pom.xml files are different. Additionally, we are not hitting the cache that was most recently produced for some reason. Therefore, sometimes with changes like the update to 2.26.0-SNAPSHOT, we are actually restoring an older version of -core that hasn't been updated yet.
Here, the change I've made just checks out the -core PR branch before checking out the main Cryostat branch so the right files are cached into the ~/.m2 folder.

[mwangggg]

demo: https://github.com/mwangggg/cryostat-core/actions/runs/7102397131/job/19332708073
So this run failed somewhere building Cryostat, but we can see it is caching the right files and compiling 2.26.0-SNAPSHOT

[andrewazores]

That failure is the same as this: https://github.com/cryostatio/cryostat/pull/1808

Cryostat (and Cryostat3) need a small update to be buildable with the latest -core release.