[CI] Push snapshots on every merge, not only on release

[andrewazores]

https://s01.oss.sonatype.org/content/repositories/snapshots/io/cryostat/cryostat-agent/

Here, there is only one snapshot available, which probably corresponds to https://github.com/cryostatio/cryostat-agent/releases/tag/early-access (?).

There should be snapshots continuously available like we do for cryostat-core: https://s01.oss.sonatype.org/content/repositories/snapshots/io/cryostat/cryostat-core/

https://github.com/cryostatio/cryostat-core/blob/87dd204478fd27eee65363949edbfa7acb11d789/.github/workflows/push-ci.yml#L31

We already have init container images that get published with each push to main, so keeping the Maven snapshots up to date as well only makes sense.

[andrewazores]

#479