fix: get tektonconfig version from label

When the `managedFields` field is present in the TektonConfig CR, the
`kubectl get tektonconfig config -o yaml` command returns multiple
`operator.tekton.dev/release-version` labels. This causes the
`tektonconfig_version_from_label` function to return an incorrect
version. This commit fixes the issue by adding a `head -n 1` command
to the `grep` command in the `tektonconfig_version_from_label` function.

test/e2e-common.sh

@@ -85,7 +85,7 @@ function current_tektonconfig_version() {
 
 function tektonconfig_version_from_label() {
   label_key='operator.tekton.dev/release-version'
-  kubectl get tektonconfig config  -o yaml | grep ${label_key} | tr -d ' ' | cut -d ':' -f 2
+  kubectl get tektonconfig config  -o yaml | grep ${label_key} | head -n 1 | tr -d ' ' | cut -d ':' -f 2
 }
 
 tektonconfig_version_from_status() {