Add a "set -e" to the dind-sidecar test script.

I was bitten by this test not having one, when the "docker build"
failed but we continued on executing anyway. I did a cursory look
at the other tests, and they either already have this or are so
short/simple that it doesn't matter.

Ref tektoncd#3628

Author: Dan Lorenc

examples/v1alpha1/taskruns/dind-sidecar.yaml

@@ -19,6 +19,7 @@ spec:
         value: /certs/client
       script: |
           #!/usr/bin/env sh
+          set -x
           # Run a Docker container.
           docker run busybox echo hello
 

examples/v1beta1/taskruns/dind-sidecar.yaml

@@ -19,6 +19,7 @@ spec:
         value: /certs/client
       script: |
           #!/usr/bin/env sh
+          set -x
           # Run a Docker container.
           docker run busybox echo hello