feat: allow disabling pushd with --no-cd

[pdecat]

🗣 Description

This PR allows to alter packer_validate behavior to not change working directory when --no-cd is passed as an argument, like it was before #47.

💭 Motivation and context

Closes #51

🧪 Testing

Tested locally with and without passing --no-cd as args, e.g.:

repos:
  - repo: https://github.com/pdecat/pre-commit-packer
    rev: feat/optional-cd
    hooks:
      - id: packer_validate
        args: [--no-cd] # disable changing working directory
      - id: packer_fmt

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All future TODOs are captured in issues, which are referenced
  in code comments.
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated
  to reflect the changes in this PR.
• Tests have been added and/or modified to cover the changes in this PR.
• All new and existing tests pass.

✅ Pre-merge checklist

• Revert dependencies to default branches.
• Finalize version.

✅ Post-merge checklist

• Create a release.

[pdecat]

export is needed to silence shellcheck SC2034.
It works without it, but shellcheck complains:

shellcheck...............................................................Failed
- hook id: shellcheck
- exit code: 1

In lib/util.sh line 23:
        NO_CD=1
        ^---^ SC2034 (warning): NO_CD appears unused. Verify use (or export if used externally).

[pdecat]

Rebased on develop to resolve conflicts

[pdecat]

Resolved linting issues, not sure how they passed local pre-hooks in the first place 😅