[Makefile] remove worker prerequisite from test_image

This is kinda revert of 1a22195 because now we rebuild
worker and test_image everytime we run check_in_container.

README does not need to be updated:
https://github.com/packit/packit-service/blob/master/CONTRIBUTING.md#running-tests-locally

Makefile

@@ -32,9 +32,12 @@ check:
 	find . -name "*.pyc" -exec rm {} \;
 	PYTHONPATH=$(CURDIR) PYTHONDONTWRITEBYTECODE=1 python3 -m pytest --color=$(COLOR) --verbose --showlocals --cov=packit_service --cov-report=$(COV_REPORT) $(TEST_TARGET)
 
-test_image: worker files/install-deps.yaml files/recipe-tests.yaml
+# first run 'make worker'
+test_image: files/install-deps.yaml files/recipe-tests.yaml
 	$(CONTAINER_ENGINE) build --rm -t $(TEST_IMAGE) -f files/docker/Dockerfile.tests --build-arg SOURCE_BRANCH=$(SOURCE_BRANCH) .
 
+# If you haven't run this for some time, run 'make worker' first.
+# 'worker' is not a dependency here because we don't want to rebuild images everytime.
 check_in_container: test_image
 	@# don't use -ti here in CI, TTY is not allocated in zuul
 	echo $(SOURCE_BRANCH)