fix(vue): missing unhead context (#3589)

Co-authored-by: Benjamin Canac <canacb1@gmail.com>

src/runtime/vue/plugins/head.ts

@@ -3,6 +3,11 @@ import type { Plugin } from 'vue'
 
 export default {
   install(app) {
+    // check for existing head instance to avoid replacement
+    // bit hacky but we can't use injectHead() here
+    if (app._context.provides.usehead) {
+      return
+    }
     const head = createHead()
     app.use(head)
   }