chore(vpa): update api-docs script

missed some exports to work correctly.

vertical-pod-autoscaler/hack/generate-api-docs.sh

@@ -33,13 +33,13 @@ if [[ -z $(which crd-ref-docs) ]]; then
         cd $WORKSPACE
         go install github.com/elastic/crd-ref-docs@latest
     )
-    CONTROLLER_GEN=${GOBIN:-$(go env GOPATH)/bin}/crd-ref-docs
+    export CONTROLLER_GEN=${GOBIN:-$(go env GOPATH)/bin}/crd-ref-docs
 else
-    CONTROLLER_GEN=$(which crd-ref-docs)
+    export CONTROLLER_GEN=$(which crd-ref-docs)
 fi
 
 cd "$(dirname "${BASH_SOURCE[0]}")/.."
-crd-ref-docs \
+$CONTROLLER_GEN \
     --source-path=pkg/apis/ \
     --config=./hack/api-docs/config.yaml \
     --renderer=markdown \