Add a script to run tasks locally in butler

[vitorguidi]

Motivation

Running a task locally is tribal knowledge for now. This PR intends to ventilate that knowledge.

Suggested vscode debug target:

{
            "name": "Debug a forced run command",
            "type": "debugpy",
            "request": "launch",
            "program": "${workspaceFolder}/butler.py",
            "console": "integratedTerminal",
            "justMyCode": false,
            "args": "run run_task --config-dir=$HOME/projects/clusterfuzz-config/configs/external --non-dry-run",
            "env": {
                "CONFIG_DIR_OVERRIDE": "/usr/local/google/home/vitorguidi/projects/clusterfuzz-config/configs/external",
                "BATCH_TEST_CONFIG_PATH": "/usr/local/google/home/vitorguidi/projects/clusterfuzz-config/configs/external",
                "DEBUG_TASK": "TRUE",
            },
        },

[jonathanmetzman]

If you add logs.configure('run_bot') then LOG_TO_CONSOLE=True causes logs to go stderr.
We should probably document other important env vars so tmp doesn't get wiped.
BOT_TMPDIR=/tmp/bot TMPDIR=/tmp/tmp