Make environment input required

There is no scenario were we wouldn't need to provide an `environment` when
manually triggering the workflow run.

Author: michalinacienciala

.github/workflows/contracts.yml

@@ -25,7 +25,7 @@ on:
     inputs:
       environment:
         description: "Environment (network) for workflow execution, e.g. `sepolia`"
-        required: false
+        required: true
       upstream_builds:
         description: "Upstream builds"
         required: false