Pin verison of ko we install 📌

We were installing ko from master @ head which means if any bugs are
introduced, we'll hit them. A bug was introduced in how -P works which
was fixed in ko-build/ko#163 by @mattmoor almost
immediately so let's pin to the version with that fix so we can deal
with changes to ko at our leisure vs. surfacing ko errors in our CI.

Fixes tektoncd#370

tekton/images/ko-gcloud/Dockerfile

@@ -22,7 +22,7 @@ ENV PATH="${PATH}:/usr/local/go/bin"
 # Install ko
 ENV GOBIN=/usr/local/go/bin
 ENV GO111MODULE on
-RUN go get github.com/google/ko/cmd/ko@master
+RUN go get github.com/google/ko/cmd/ko@v0.5.0
 
 # Install Kustomize
 RUN go get sigs.k8s.io/kustomize/kustomize/[email protected]