fix: ensure we are talking to coder on first user check (#11130)

codersdk/users.go

@@ -190,7 +190,15 @@ func (c *Client) HasFirstUser(ctx context.Context) (bool, error) {
 		return false, err
 	}
 	defer res.Body.Close()
+
 	if res.StatusCode == http.StatusNotFound {
+		// ensure we are talking to coder and not
+		// some other service that returns 404
+		v := res.Header.Get("X-Coder-Build-Version")
+		if v == "" {
+			return false, xerrors.Errorf("missing build version header, not a coder instance")
+		}
+
 		return false, nil
 	}
 	if res.StatusCode != http.StatusOK {