Ignore bundled gems for Rubocop and ESLint in GitHub Actions

Rails projects will often install gems into the vendor directory in GitHub actions so we can cache them for the consecutive runs.

If you use code linters in the action you need to exclude the `vendor/bundle/*` to prevent false positives

For ESLint this can be configured through ignorePatterns

// ./.eslintrc.json
{
  "ignorePatterns": [
    "vendor/bundle/*"
  ]
}

For Rubocop you can exclude through the AllCops config

# ./.rubocop.yml
AllCops:
  Exclude:
    - 'vendor/**/*'