diceware: switch to buildString lambda

passgen/diceware/src/main/kotlin/dev/msfjarvis/aps/passgen/diceware/DicewarePassphraseGenerator.kt

@@ -23,15 +23,12 @@ constructor(
 
   /** Generates a passphrase with [wordCount] words. */
   public fun generatePassphrase(wordCount: Int, separator: Char): String {
-    return StringBuilder()
-      .apply {
+    return buildString {
       repeat(wordCount) { idx ->
         append(wordMap[die.rollMultiple(DIGITS)])
         if (idx < wordCount - 1) append(separator)
       }
     }
-      .toString()
-      .trimEnd()
   }
 
   private companion object {