Update make targets: (#1900)

* Update make targets:

* `make dev` now shows a message indicating the new commands
* `make build` builds the binary into bin/
* `make install` will subsequently `mv` (not copy) the binary into
  GOPATH/bin

The reason for the `mv` is that M1 Macs seem to get pretty unhappy about
executing a copied binary if it's been modified, so an initial binary is
fine, then after changing and recompiling the command keeps getting
killed.

* Make the 'make install' text more clear

Makefile

@@ -29,12 +29,23 @@ cleangen:
 	@rm -f ${GENERATED_CODE}
 
 .PHONY: dev
-dev: BUILD_TAGS+=dev
-dev: BUILD_TAGS+=ui
-dev: build-ui-ifne
-	@echo "==> Building Boundary with dev and UI features enabled"
+dev:
+	@echo "This command has changed. Please use:"
+	@echo "==> make build"
+	@echo "      to build the binary into the bin/ directory"
+	@echo "==> make install"
+	@echo "      to build the binary and install it into GOPATH/bin"
+
+.PHONY: build
+build: BUILD_TAGS+=ui
+build: build-ui-ifne
+	@echo "==> Building Boundary with UI features enabled"
 	@CGO_ENABLED=$(CGO_ENABLED) BUILD_TAGS='$(BUILD_TAGS)' sh -c "'$(CURDIR)/scripts/build.sh'"
 
+.PHONY: install
+install: export BOUNDARY_INSTALL_BINARY=1
+install: build
+
 .PHONY: fmt
 fmt:
 	gofumpt -w $$(find . -name '*.go' | grep -v pb.go | grep -v pb.gw.go)