Use Tekton's nightly-built build-base image

Apparently the knative-nightly build-base image hasn't been built since
February?!

.ko.yaml

@@ -1,7 +1,6 @@
 baseImageOverrides:
-  # TODO(christiewilson): Use our built base image
-  github.com/tektoncd/pipeline/cmd/creds-init: gcr.io/knative-nightly/github.com/knative/build/build-base:latest
-  github.com/tektoncd/pipeline/cmd/git-init: gcr.io/knative-nightly/github.com/knative/build/build-base:latest
+  github.com/tektoncd/pipeline/cmd/creds-init: gcr.io/tekton-nightly/github.com/tektoncd/pipeline/build-base:latest
+  github.com/tektoncd/pipeline/cmd/git-init: gcr.io/tekton-nightly/github.com/tektoncd/pipeline/build-base:latest
   github.com/tektoncd/pipeline/cmd/bash: busybox # image should have shell in $PATH
   github.com/tektoncd/pipeline/cmd/entrypoint: busybox # image should have shell in $PATH
   github.com/tektoncd/pipeline/cmd/gsutil: google/cloud-sdk:alpine # image should have gsutil in $PATH

test/git_checkout_test.go

@@ -126,7 +126,7 @@ func TestGitPipelineRunFail(t *testing.T) {
 								t.Fatalf("Error getting pod logs for pod `%s` and container `%s` in namespace `%s`", tr.Status.PodName, stat.Name, namespace)
 							}
 							// Check for failure messages from fetch and pull in the log file
-							if strings.Contains(string(logContent), "Couldn't find remote ref Idontexistrabbitmonkeydonkey") &&
+							if strings.Contains(string(logContent), "couldn't find remote ref Idontexistrabbitmonkeydonkey") &&
 								strings.Contains(string(logContent), "pathspec 'Idontexistrabbitmonkeydonkey' did not match any file(s) known to git") {
 								t.Logf("Found exepected errors when retrieving non-existent git revision")
 							} else {