static void setSigmaAndKey(int[] state, final int[] key) {
    System.arraycopy(SIGMA, 0, state, 0, SIGMA.length); // 4 words
    System.arraycopy(key, 0, state, SIGMA.length, KEY_SIZE_IN_INTS); // 8 words
  }
Sets the first 12 words of the initial state as described in
https://datatracker.ietf.org/doc/html/rfc7539#section-2.3.
java_src/src/main/java/com/google/crypto/tink/aead/internal/ChaCha20Util.java