ci: Enable concurrency in pre-staging deploy workflow

[ppawlowski]

Description

This pull request enables the concurrency feature within Create fresh app instance for PR workflow. This change assures, that only one workflow for particular PR will be executed at the same time. This will fix a case for orphaned pre-staging environments created for short-lived pull requests.

Related Issue(s)

Checklist

• [x] I have read the contribution guidelines
• [ ] Suitable unit/system level tests have been added and they pass
• [ ] Documentation has been updated
  • [ ] Upgrade instructions
  • [ ] Configuration details
  • [ ] Concepts
• [ ] Changes flowforge.yml?
  • [ ] Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • [ ] Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production

Labels

• [ ] Includes a DB migration? -> add the area:migration label