Make pushd in packer_validate hook optional

# 🐛 Summary #

Since https://github.com/cisagov/pre-commit-packer/pull/47, the current working directory is changed to the location of each HCL template.

This breaks setups where HCL templates are in subdirectories and relatively include ansible playbooks, e.g.:

```
.
├── Makefile
├── ansible
│   ├── ansible.cfg
│   ├── group_vars
│   │   └── all.yml
│   ├── playbook.yml
│   └── requirements.yml
├── packer
│   └── main.pkr.hcl
├── renovate.json
└── requirements.txt
```

With `packer/main.pkr.hcl` containing:
```hcl
build {
provisioner "ansible" {
playbook_file = "./ansible/playbook.yml"
}
}
```

And `.pre-commit-config.yaml`:
```
repos:
- repo: https://github.com/cisagov/pre-commit-packer
rev: v0.3.0
hooks:
- id: packer_validate
- id: packer_fmt
```

As I understand the motivation behind the change, e.g. for monorepos containing multiple independent packer templates, it would be awesome to make this configurable.

## To reproduce ##

Steps to reproduce the behavior:

1. Create the above setup
1. Then run `pre-commit run --all-files packer_validate`

## Expected behavior ##

Hook should pass, just like a normal invocation:
```sh
# packer validate packer/main.pkr.hcl
The configuration is valid.
```

## Any helpful log output or screenshots ##

Paste the results here:

```console
# pre-commit run --all-files packer_validate
Packer Validate..........................................................Failed
- hook id: packer_validate
- exit code: 1

Error: Failed preparing provisioner-block "ansible" ""

on main.pkr.hcl line 84:
(source code not available)

1 error(s) occurred:

* playbook_file: ./ansible/playbook.yml is invalid: stat ./ansible/playbook.yml:
no such file or directory

Failed path: packer
================================
```