fix: Invalid label checking in CI

[alexbenedicto]

After looking at #142 actions, the "test-geos-integration" label used was not found when reaching the "geos_ci_dispatch" job.

Two problems fixed:

1. The old code tried to iterate over a JSON array as a string, which meant it only looped once with the entire JSON as a single item.
2. github.event.pull_request.labels returns an array of label objects with properties like name, color, id, etc even though we were only looking at names.

A small refactor to use variables for the label names was also added.

[jafranc]

Good catch !