key_manager: Use isxdigit instead of isdigit when reading key file

Crypto revisions are hex numbers and this function only checks if the string is valid for stoul in base 16, so it should be isxdigit.

src/core/crypto/key_manager.cpp

@@ -395,7 +395,7 @@ static bool ValidCryptoRevisionString(std::string_view base, size_t begin, size_
     if (base.size() < begin + length)
         return false;
     return std::all_of(base.begin() + begin, base.begin() + begin + length,
-                       [](u8 c) { return std::isdigit(c); });
+                       [](u8 c) { return std::isxdigit(c); });
 }
 
 void KeyManager::LoadFromFile(const std::string& filename, bool is_title_keys) {