Remove -mod=readonly from the helloworld's Dockerfile (#1017)

The go.mod file was removed in
4ea31c4
, and breaks building using Tekton.

The error is as follows:
STEP 3: COPY ./test/test_images/helloworld/ .
STEP 4: RUN CGO_ENABLED=0 GOOS=linux go build -mod=readonly -v -o server
build flag -mod=readonly only valid when using modules
subprocess exited with status 1
subprocess exited with status 1
error building at STEP "RUN CGO_ENABLED=0 GOOS=linux go build
-mod=readonly -v -o server": exit status 1

test/test_images/helloworld/Dockerfile

@@ -10,8 +10,7 @@ WORKDIR /app
 COPY ./test/test_images/helloworld/ .
 
 # Build the binary.
-# -mod=readonly ensures immutable go.mod and go.sum in container builds.
-RUN CGO_ENABLED=0 GOOS=linux go build -mod=readonly -v -o server
+RUN CGO_ENABLED=0 GOOS=linux go build -v -o server
 
 # Use the official Alpine image for a lean production container.
 # https://hub.docker.com/_/alpine