ci(image): correct expression condition syntax

[andrewazores]

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Related to #144

https://github.com/cryostatio/cryostat3/actions/runs/6791664907/job/18463577147

I believe this failed because of the curly brace syntax. In my fork this was commented out to make sure it progressed to the "push to quay.io" stage and then failed as expected. Uncommenting this condition resulted in the JDK 21 build attempting to build the multiarch manifest image when it should have skipped that step.