Update sonobuoy version for v1.20.0

Also drop the rancher-mirrored sonobuoy image since CI has a
pull-through image cache now.

Signed-off-by: Brad Davidson <brad.davidson@rancher.com>

Dockerfile.test.dapper

@@ -2,7 +2,7 @@ FROM golang:1.15.5-alpine3.12
 
 RUN apk -U --no-cache add bash git gcc musl-dev docker curl jq coreutils python2 openssl py-pip
 
-ENV SONOBUOY_VERSION 0.19.0
+ENV SONOBUOY_VERSION 0.20.0
 
 RUN OS=linux; \
     ARCH=$(go env GOARCH); \

scripts/test-helpers

@@ -234,7 +234,6 @@ sonobuoy-test() {
     sonobuoy run \
         --config=scripts/sonobuoy-config.json \
         --plugin-env=e2e.E2E_USE_GO_RUNNER=true \
-        --sonobuoy-image=rancher/sonobuoy-sonobuoy:v${SONOBUOY_VERSION:-0.19.0} \
         --kube-conformance-image-version=${VERSION_K8S} \
         --wait=90 \
         $@ &