[ci] don't use variables in workflow

I don't understand them

.woodpecker/workflow.yaml

@@ -10,10 +10,6 @@ matrix:
     - 3.13
     # - 3.12
 
-variables:
-  - UV_VERSION: "latest"
-  - UV_BASE_LAYER: "trixie-slim"
-
 steps:
   - name: Lint python code
     image: pipelinecomponents/ruff
@@ -26,7 +22,7 @@ steps:
       - markdownlint-cli2 .
 
   - name: Run Tests
-    image: ghcr.io/astral-uv/uv:${UV_VERSION}-python${PYTHON_VERSION}-${UV_BASE_LAYER}
+    image: ghcr.io/astral-uv/uv:latest-python${PYTHON_VERSION}-trixie-slim
     commands:
       - uv install
       - uv run pytest tests.py