Log the executed spawns into this file as length-delimited SpawnExec protos, according to src/main/protobuf/spawn.proto. Prefer --execution_log_compact_file, which is significantly smaller and cheaper to produce. Related flags: --execution_log_compact_file (compact format; mutually exclusive), --execution_log_json_file (text JSON format; mutually exclusive), --execution_log_sort (whether to sort the execution log), --subcommands (for displaying subcommands in terminal output).