Fix GitHub Actions workflow: Update to use existing Docker images

Problem:
- Workflow was failing because nytimes/blender Docker images don't exist for versions 4.2 and 5.0
- Error: 'manifest for nytimes/blender:5.0-cpu-ubuntu22.04 not found'
- nytimes/blender repository only has images up to version 3.3.1

Solution:
- Updated all jobs to use blenderkit/headless-blender images
- Blender 5.0: nytimes/blender:5.0-cpu-ubuntu22.04 → blenderkit/headless-blender:blender-5.0
- Blender 4.2: nytimes/blender:4.2-cpu-ubuntu22.04 → blenderkit/headless-blender:blender-4.2

BlenderKit headless-blender images:
- Actively maintained with versions 2.93 through 5.1
- Designed for headless CI/CD workflows
- Available at: https://hub.docker.com/r/blenderkit/headless-blender
- Source: https://github.com/BlenderKit/headless-blender-container

All three workflow jobs updated:
- test-blender-5 (Test with Blender 5.0)
- test-blender-4 (Test with Blender 4.2 LTS)
- quick-check (Quick smoke test)

Author: zbuc

.github/workflows/blender-tests.yml

@@ -16,7 +16,7 @@ jobs:
     name: Test with Blender 5.0
     runs-on: ubuntu-latest
     container:
-      image: nytimes/blender:5.0-cpu-ubuntu22.04
+      image: blenderkit/headless-blender:blender-5.0
 
     steps:
       - name: Checkout code
@@ -61,7 +61,7 @@ jobs:
     name: Test with Blender 4.2 (LTS)
     runs-on: ubuntu-latest
     container:
-      image: nytimes/blender:4.2-cpu-ubuntu22.04
+      image: blenderkit/headless-blender:blender-4.2
 
     steps:
       - name: Checkout code
@@ -98,7 +98,7 @@ jobs:
     name: Quick smoke test (pre-merge)
     runs-on: ubuntu-latest
     container:
-      image: nytimes/blender:5.0-cpu-ubuntu22.04
+      image: blenderkit/headless-blender:blender-5.0
 
     steps:
       - name: Checkout code