Update PR workflow file

The variable broke the workflow because the GitHub Actions containers do
not have Docker executable in the default location set in the Makefile.

Signed-off-by: Aitor Perez Cedres <[email protected]>

Author: Zerpet

.github/workflows/pr.yml

@@ -40,5 +40,5 @@ jobs:
         kind create cluster --image kindest/node:"$K8S_VERSION"
         make cert-manager
         make cluster-operator
-        DOCKER_REGISTRY_SERVER=local-server OPERATOR_IMAGE=local-operator make deploy-kind
-        make system-tests
+        DOCKER_REGISTRY_SERVER=local-server OPERATOR_IMAGE=local-operator make deploy-kind BUILD_KIT=docker
+        make system-tests BUILD_KIT=docker