Replace Str::random() call with simple inline generator

The use of `Str::random()` within the validator can lead to
non-deterministic test outcomes when using random string mocking such as
`Str::createRandomStringsUsing()` or
`Str::createRandomStringsUsingSequence()`.

This simple inline generator needn't be cryptographically secure in this
context as we only need a unique placeholder string.

Author: michaeldyrynda

src/Illuminate/Validation/Validator.php

@@ -345,7 +345,7 @@ public function __construct(
         array $attributes = [],
     ) {
         if (! isset(static::$placeholderHash)) {
-            static::$placeholderHash = Str::random();
+            static::$placeholderHash = substr(str_shuffle('0123456789abcdefghijklmnopqrstuvwxyz'), 0, 16);
         }
 
         $this->initialRules = $rules;