Makefile: Fix setting PATH

Running `sudo -E PATH=... foo` doesn't actually use the provided PATH to
lookup foo, it just sets PATH before execing the foo found by sudo
(which doesn't look in the provided PATH).

Signed-off-by: Lewis Marshall <lewis@lmars.net>

Makefile

@@ -12,10 +12,10 @@ clean:
 test: test-unit test-integration
 
 test-unit: build
-	$(GO_ENV) PATH=${PWD}/build/bin:${PATH} go test -race -cover ./...
+	env $(GO_ENV) PATH=${PWD}/build/bin:${PATH} go test -race -cover ./...
 
 test-unit-root: test-unit
-	sudo -E $(GO_ENV) PATH=${PWD}/build/bin:${PATH} go test -race -cover ./host/volume/...
+	sudo -E env $(GO_ENV) PATH=${PWD}/build/bin:${PATH} go test -race -cover ./host/volume/...
 
 test-integration: build
 	script/run-integration-tests