Support Running Without GitHub Token/Integration

[naftulikay]

Motivation

Setting up a GitHub token usually isn't easy and often requires access that I, as a member of an organization, don't have. Presently, if token is not passed, the job fails. All I'd like it to do is to run cargo audit without needing to access/update anything in GitHub. I simply want my build to fail with reports on what vulnerabilities were found in the logs within GitHub Actions.

Workflow example

jobs:
  audit:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v1
      - uses: actions-rs/audit-check@v1

[naftulikay]

Duplicate of #180.