fix: pass env var PUSH_TEST_IMAGES to child pipeline

Closes #101 (closed)

How to verify:

1. Go to Pipelines
2. Click 'New Pipeline'
3. Select branch "101-fix-push-test-images-var"
4. Provide two env vars: PUSH_TEST_IMAGES=True and FORCE_DYNAMIC_JOBS=True
5. Open any dynamic job, you should see something similar in logs

+ summary+='Branch '\''101-fix-push-test-images-var'\'' is not '\''master'\'' and PUSH_TEST_IMAGES is '\''True'\'' so pushing to '\''test'\''.\n'
+ image_folder=registry.gitlab.developers.cam.ac.uk/uis/devops/infra/dockerimages/test