Fix (make vendor)

(export a=b command args) does not run (command args) with a=b,
it sets $a to b, and marks variables $a $command $args as exported,
i.e. (command args) is not run.

So we were not actually running (go mod tidy).

Fix that.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

Makefile

@@ -637,10 +637,9 @@ build-no-cgo:
 
 .PHONY: vendor
 vendor:
-	export GO111MODULE=on \
-		$(GO) mod tidy && \
-		$(GO) mod vendor && \
-		$(GO) mod verify
+	GO111MODULE=on $(GO) mod tidy
+	GO111MODULE=on $(GO) mod vendor
+	GO111MODULE=on $(GO) mod verify
 
 .PHONY: vendor-in-container
 vendor-in-container: