Use vendor folder to speed up CI 🥄

We still are using the `vendor` folder, so the CI should use this to
build instead of trying to download the world.

- It will speed up the CI
- It will make sure we can build and test with what's in `vendor`

This also updates release Task(s) and Pipeline(s) to use the `vendor` folder.

Signed-off-by: Vincent Demeester <vdemeest@redhat.com>

tekton/publish.yaml

@@ -111,6 +111,8 @@ spec:
       value: /workspace/go
     - name: GO111MODULE
       value: "off"
+    - name: GOFLAGS
+      value: "-mod=vendor"
     - name: CLOUDSDK_AUTH_CREDENTIAL_FILE_OVERRIDE
       value: /secret/release.json
     script: |

tekton/release-pipeline.yaml

@@ -76,6 +76,8 @@ spec:
       params:
         - name: package
           value: $(params.package)
+        - name: flags
+          value: -v -mod=vendor
       resources:
         inputs:
           - name: source
@@ -88,7 +90,7 @@ spec:
         - name: package
           value: $(params.package)
         - name: flags
-          value: -ldflags "-X github.com/tektoncd/pipeline/pkg/version.PipelineVersion=$(params.versionTag)"
+          value: -mod=vendor -ldflags "-X github.com/tektoncd/pipeline/pkg/version.PipelineVersion=$(params.versionTag)"
       resources:
         inputs:
           - name: source