fix missing import

Author: lefou

runner/server/client/src/mill/client/ClientUtil.java

@@ -2,6 +2,7 @@
 
 import java.io.*;
 import java.nio.charset.Charset;
+import java.nio.charset.StandardCharsets;
 import java.nio.file.Path;
 import java.util.*;
 import java.util.regex.Matcher;