Removes time measurement from the Makefile (flutter#9305)

`time` is not a valid command on Windows. This helps to setup the
environment on the Windows platform. We currently not monitoring the
built time so we can add it back once we have requests.

Issues fixed (partially) by this PR:

- flutter#6443
- flutter#8272

Makefile

@@ -89,7 +89,7 @@ emulate:
 # WARNING this can take a while to run!
 # Usage: `make test FLUTTER_TEST_BRANCH=<channel>`
 test:
-	DOCKER_BUILDKIT=1 time docker build --rm --target tests -t flt-test .
+	DOCKER_BUILDKIT=1 docker build --rm --target tests -t flt-test .
 	docker run --rm --name flt-tests -t flt-test --target ${FLUTTER_TEST_BRANCH}
 
 # Stop long running tests
@@ -110,13 +110,13 @@ debug-tests:
 # those will be run on the Github action.
 # Usage: `make build-image`
 build-image:
-	DOCKER_BUILDKIT=1 time docker build --rm --no-cache --target build \
+	DOCKER_BUILDKIT=1 docker build --rm --no-cache --target build \
 		--build-arg BUILD_CONFIGS=${BUILD_CONFIGS} -t ${BUILD_TAG} .
 
 # Build the production site & Run the link checker
 # Usage: `make check-links`
 build-image-and-check-links:
-	DOCKER_BUILDKIT=1 time docker build --rm --no-cache --target checklinks \
+	DOCKER_BUILDKIT=1 docker build --rm --no-cache --target checklinks \
 		--build-arg BUILD_CONFIGS=${BUILD_CONFIGS} -t ${BUILD_TAG} .
 	docker run --rm -t ${BUILD_TAG}