fix: create .ssh folder if it does not exist already when running `coder config-ssh`

A user ran into the following error when running `coder config-ssh`

```
Encountered an error running "coder config-ssh", see "coder config-ssh --help" for more information
error: write ssh config failed: cannot create temp file: .ssh/config: no such file or directory
```

We should be creating the directory if it does not exist.