Error: EISDIR when webhook event is triggered

## Problem Description
Self-hosted deployment of safe settings throwing the following error when a webhook event is received:

`AggregateError:
Error: EISDIR: illegal operation on a directory, read
at loadYamlFileSystem (/opt/safe-settings/index.js:100:41)
at syncSettings (/opt/safe-settings/index.js:67:32)
at /opt/safe-settings/index.js:265:12
at /opt/safe-settings/node_modules/@octokit/webhooks/dist-node/index.js:177:14
at async Promise.all (index 0)
at async middleware (/opt/safe-settings/node_modules/@octokit/webhooks/dist-node/index.js:355:5)`

### What is actually happening
Nothing happens, error seems to be blocking the deploymentConfig from being read.

When listing the `/app/` directory in deployed container, instead of a `deployment-config.yaml` file it is set as a directory.

### What is the expected behavior
`/app/deployment-config.yaml` would be set as a file instead of a directory and safe settings application would receive the webhook event and apply the settings from the `admin` repository.

### Error output, if available

```
AggregateError:
Error: EISDIR: illegal operation on a directory, read
at loadYamlFileSystem (/opt/safe-settings/index.js:100:41)
at syncSettings (/opt/safe-settings/index.js:67:32)
at /opt/safe-settings/index.js:265:12
at /opt/safe-settings/node_modules/@octokit/webhooks/dist-node/index.js:177:14
at async Promise.all (index 0)
at async middleware (/opt/safe-settings/node_modules/@octokit/webhooks/dist-node/index.js:355:5)
```

## Context

### Are you using the hosted instance of probot/settings or running your own?
Running our own instance utilizing the helm chart described in the Deployment docs.

### If running your own instance, are you using it with github.com or GitHub Enterprise?
github.com

#### Version of probot/settings
v2.0.17

#### Version of GitHub Enterprise