Makefile: run "go mod tidy" when vendoring

`make vendor` did not do a `go mod tidy` after vendoring, whereas
CI does this as part of the vendor validation, causing CI to fail
if `make vendor` was used to update vendoring, without doing a
`go mod tidy` as well.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Makefile

@@ -376,6 +376,7 @@ root-coverage: ## generate coverage profiles for unit tests that require root
 
 vendor:
 	@echo "$(WHALE) $@"
+	@go mod tidy
 	@go mod vendor
 
 help: ## this help