Run yarn audit only in beginning of CircleCI

[yhatt]

Sometime CircleCI fails by returning error in yarn audit with 503 status code even if test has not any problems.

#!/bin/bash -eo pipefail
yarn audit

yarn audit v1.13.0
/opt/yarn-v1.13.0/lib/cli.js:66237
            throw new (_errors || _load_errors()).ResponseError(_this3.reporter.lang('requestFailed', description), res.statusCode);
            ^

Error: Request failed "503 Service Unavailable"
    at ResponseError.ExtendableBuiltin (/opt/yarn-v1.13.0/lib/cli.js:702:66)
    at new ResponseError (/opt/yarn-v1.13.0/lib/cli.js:808:124)
    at Request.params.callback [as _callback] (/opt/yarn-v1.13.0/lib/cli.js:66237:19)
    at Request.self.callback (/opt/yarn-v1.13.0/lib/cli.js:129397:22)
    at Request.emit (events.js:189:13)
    at Request.<anonymous> (/opt/yarn-v1.13.0/lib/cli.js:130369:10)
    at Request.emit (events.js:189:13)
    at IncomingMessage.<anonymous> (/opt/yarn-v1.13.0/lib/cli.js:130291:12)
    at Object.onceWrapper (events.js:277:13)
    at IncomingMessage.emit (events.js:194:15)
Exited with code 1

In background, 3 audit requests are running at the same time. They may be overloading npm's API. So I updated CircleCI workflow to run only yarn audit at first.

It would give a feedback to the other Marp Team projects too.