Add tidy to the verify target (#320)

As part of verifying, we should really be checking the status of
`go mod tidy`. This can be done by adding the `tidy` target as a
dependency of the `verify` target. This will be ussed during
github workflows/CI to make sure everything is "tidied".

Makefile

@@ -82,7 +82,7 @@ TEST_PKGS:=$(shell go list ./...)
 test-unit: ## Run the unit tests
 	$(Q)go test -count=1 -short ${TEST_PKGS}
 
-verify: manifests generate format
+verify: manifests generate format tidy
 	git diff --exit-code
 
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