Fixed clean target in Makefile

As there are no longer any Go files in the project root we need to
point go clean towards something that does contain Go files. Not doing
this will result in an error:

  $ go clean
  can't load package: package github.com/fluxcd/helm-operator: unknown
  import path "github.com/fluxcd/helm-operator": cannot find module
  providing package github.com/fluxcd/helm-operator

Makefile

@@ -30,7 +30,7 @@ DOCS_PORT:=8000
 all: $(GOBIN)/bin/helm-operator build/.helm-operator.done
 
 clean:
-	go clean
+	go clean ./cmd/helm-operator
 	rm -rf ./build
 	rm -f test/bin/kubectl test/bin/helm test/bin/kind