Speed up all make targets by only finding generated files when we need to (#1937)

Makefile

@@ -6,8 +6,6 @@ THIS_DIR := $(dir $(realpath $(firstword $(MAKEFILE_LIST))))
 TMP_DIR := $(shell mktemp -d)
 REPO_PATH := github.com/hashicorp/boundary
 
-GENERATED_CODE := $(shell  find ${THIS_DIR} -name '*.gen.go' && find ${THIS_DIR} -name '*.pb.go' && find ${THIS_DIR} -name '*.pb.gw.go')
-
 CGO_ENABLED?=0
 
 export GEN_BASEPATH := $(shell pwd)
@@ -26,7 +24,7 @@ tools:
 
 .PHONY: cleangen
 cleangen:
-	@rm -f ${GENERATED_CODE}
+	@rm -f $(shell  find ${THIS_DIR} -name '*.gen.go' && find ${THIS_DIR} -name '*.pb.go' && find ${THIS_DIR} -name '*.pb.gw.go')
 
 .PHONY: install-no-plugins
 install-no-plugins: export SKIP_PLUGIN_BUILD=1