Use vendor-ed version of plumbing instead of go get 🦸

Using `go get -d` updates continuously the plumbing dependency,
without really updating the vendor folder, which means it becomes
inconsistency.

This is a follow-up of tektoncd#1763.

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

test/e2e-common.sh

@@ -16,8 +16,7 @@
 
 # Helper functions for E2E tests.
 
-go get -d github.com/tektoncd/plumbing
-source $(go list -m -f '{{.Dir}}' github.com/tektoncd/plumbing)/scripts/e2e-tests.sh
+source $(dirname $0)/../vendor/github.com/tektoncd/plumbing/scripts/e2e-tests.sh
 
 function teardown() {
     subheader "Tearing down Tekton Pipelines"

test/presubmit-tests.sh

@@ -26,8 +26,7 @@
 export DISABLE_MD_LINTING=1
 export DISABLE_MD_LINK_CHECK=1
 
-go get -d github.com/tektoncd/plumbing
-source $(go list -m -f '{{.Dir}}' github.com/tektoncd/plumbing)/scripts/presubmit-tests.sh
+source $(dirname $0)/../vendor/github.com/tektoncd/plumbing/scripts/e2e-tests.sh
 
 function post_build_tests() {
   header "running golangci-lint"