chore: ensure proper rbac permissions on 'Acquire' file in the cache (#18348)

The file cache was caching the `Unauthorized` errors if a user without
the right perms opened the file first. So all future opens would fail.

Now the cache always opens with a subject that can read files. And authz
is checked on the Acquire per user.

coderd/users.go

@@ -525,7 +525,7 @@ func (api *API) deleteUser(rw http.ResponseWriter, r *http.Request) {
 	ctx := r.Context()
 	auditor := *api.Auditor.Load()
 	user := httpmw.UserParam(r)
-	auth := httpmw.UserAuthorization(r)
+	auth := httpmw.UserAuthorization(r.Context())
 	aReq, commitAudit := audit.InitRequest[database.User](rw, &audit.RequestParams{
 		Audit:   auditor,
 		Log:     api.Logger,