Properly escape $ in shell command in Makefile
# limitations under the License.
IMG = gcr.io/knative-tests/test-infra/prow-tests
-TAG := $(shell date +v%Y%m%d)-$(shell git describe --always --dirty --match '^$')
+TAG := $(shell date +v%Y%m%d)-$(shell git describe --always --dirty --match '^$$')
all: build
