Default for persist-credentials should be false

[sebbASF]

Does not seem right to expose the credentials by default.

[sebbASF]

Note: if this PR is not accepted, then input-helper.ts should probably be fixed to remove the inconsistent default:

https://github.com/actions/checkout/blob/8230315d06ad95c617244d2f265d237a1682d445/src/input-helper.ts#L120