make: add phony to build

This is a convenience to get the makefile to build whether it likes it or not.

src/go/rpk/Makefile

@@ -29,6 +29,7 @@ all: help
 
 ready: build test fmt lint bazel check_diff ## Runs all. Ensures commit is ready.
 
+.PHONY: build
 build: ## Build rpk.
 	$(shell mkdir -p $(OUTDIR))
 	$(GOCMD) build -ldflags '$(LDFLAGS)' -o $(OUTDIR) ./...