gha: Skip duplicate image build on merge

[BbolroC]

The following workflows are triggered twice during a merge:

• build-kbs-image.yml
• build-as-image.yml

For self-hosted runners, which often have a limited number of available runners, this duplicate build is unnecessary and may cause CI bottlenecks.

This PR ensures the workflow of interest is skipped when github.event_name is push, which occurs during a merge.

Signed-off-by: Hyounggyu Choi Hyounggyu.Choi@ibm.com

[BbolroC]

Workflow examples for the duplicate build:

1. KBS

   • https://github.com/confidential-containers/trustee/actions/runs/11855132932
   • https://github.com/confidential-containers/trustee/actions/runs/11855132934

2. AS

   • https://github.com/confidential-containers/trustee/actions/runs/11855132938
   • https://github.com/confidential-containers/trustee/actions/runs/11855132944