Add check before creating task

Tasks with when expressions using variables that evaluated to false were
mistakenly were mistakenly executed because we missed an additional check
to validate that the task should be skipped after variable replacement.

Fixes tektoncd#3188

examples/v1beta1/pipelineruns/pipelinerun-with-when-expressions.yaml

@@ -66,6 +66,16 @@ spec:
           - name: echo
             image: ubuntu
             script: 'echo file exists'
+    - name: echo-file-missing
+      when:
+        - input: "$(tasks.check-file.results.status)"
+          operator: in
+          values: ["missing"]
+      taskSpec:
+        steps:
+          - name: echo
+            image: ubuntu
+            script: 'echo file missing'
     - name: task-should-be-skipped
       when:
         - input: "$(params.path)"

pkg/reconciler/pipelinerun/pipelinerun.go

@@ -522,7 +522,7 @@ func (c *Reconciler) runNextSchedulableTask(ctx context.Context, pr *v1beta1.Pip
 	resources.ApplyTaskResults(nextRprts, resolvedResultRefs)
 
 	for _, rprt := range nextRprts {
-		if rprt == nil {
+		if rprt == nil || rprt.Skip(pipelineState, d) {
 			continue
 		}
 		if rprt.ResolvedConditionChecks == nil || rprt.ResolvedConditionChecks.IsSuccess() {