message: refactor: rename scriptPubKey in VerifyWitnessProgram to exec_script
  The old name is confusing, as it doesn't store a scriptPubKey, but the
  actually executed script.
diff: @@ -1532,7 +1532,7 @@ static bool ExecuteWitnessScript(const Span<const valtype>& stack_span, const CS
  static bool VerifyWitnessProgram(const CScriptWitness& witness, int witversion, const std::vector<unsigned char>& program, unsigned int flags, const BaseSignatureChecker& checker, ScriptError* serror)
  {
  - CScript scriptPubKey;
  + CScript exec_script; //!< Actually executed script (last stack item in P2WSH; implied P2PKH script in P2WPKH)
  Span<const valtype> stack{witness.stack};
  if (witversion == 0) {
  @@ -1542,20 +1542,20 @@ static bool VerifyWitnessProgram(const CScriptWitness& witness, int witversion,
  return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_WITNESS_EMPTY);
  }
  const valtype& script_bytes = SpanPopBack(stack);
  - scriptPubKey = CScript(script_bytes.begin(), script_bytes.end());
  - uint256 hashScriptPubKey;
  - CSHA256().Write(&scriptPubKey[0], scriptPubKey.size()).Finalize(hashScriptPubKey.begin());
  - if (memcmp(hashScriptPubKey.begin(), program.data(), 32)) {
  + exec_script = CScript(script_bytes.begin(), script_bytes.end());
  + uint256 hash_exec_script;
  + CSHA256().Write(&exec_script[0], exec_script.size()).Finalize(hash_exec_script.begin());
  + if (memcmp(hash_exec_script.begin(), program.data(), 32)) {
  return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_MISMATCH);
  }
  - return ExecuteWitnessScript(stack, scriptPubKey, flags, SigVersion::WITNESS_V0, checker, serror);
  + return ExecuteWitnessScript(stack, exec_script, flags, SigVersion::WITNESS_V0, checker, serror);
  } else if (program.size() == WITNESS_V0_KEYHASH_SIZE) {
  // Special case for pay-to-pubkeyhash; signature + pubkey in witness
  if (stack.size() != 2) {
  return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_MISMATCH); // 2 items in witness
  }
  - scriptPubKey << OP_DUP << OP_HASH160 << program << OP_EQUALVERIFY << OP_CHECKSIG;
  - return ExecuteWitnessScript(stack, scriptPubKey, flags, SigVersion::WITNESS_V0, checker, serror);
  + exec_script << OP_DUP << OP_HASH160 << program << OP_EQUALVERIFY << OP_CHECKSIG;
  + return ExecuteWitnessScript(stack, exec_script, flags, SigVersion::WITNESS_V0, checker, serror);
  } else {
  return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_WRONG_LENGTH);
  }
  