Flush partialCached cache on rebuilds

Fixes #4931

hugolib/hugo_sites_build.go

@@ -28,6 +28,7 @@ import (
 // Build builds all sites. If filesystem events are provided,
 // this is considered to be a potential partial rebuild.
 func (h *HugoSites) Build(config BuildCfg, events ...fsnotify.Event) error {
+
 	if h.Metrics != nil {
 		h.Metrics.Reset()
 	}
@@ -42,6 +43,10 @@ func (h *HugoSites) Build(config BuildCfg, events ...fsnotify.Event) error {
 		conf.whatChanged = &whatChanged{source: true, other: true}
 	}
 
+	for _, s := range h.Sites {
+		s.Deps.BuildStartListeners.Notify()
+	}
+
 	if len(events) > 0 {
 		// Rebuild
 		if err := h.initRebuild(conf); err != nil {