Add a backup path to look for the OPA executable in the current directory

[buidav]

🗣 Description

• Add a backup path to RunRego.psm1 that checks the user's current executing directory if ScubaGear fails to detect the OPA executable in the .scubagear/tools folder.
• This is to assist with troubleshooting when the OPA executable fails to download properly.

💭 Motivation and context

Closes #835

🧪 Testing

• Remove the opa executable from the tools folder
• Add a copy to your current executing directory.
• If a copy is not found in both ScubaGear will throw a more detailed error message.

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• PR targets the correct parent branch (e.g., main or release-name) for merge.
• Changes are limited to a single goal - eschew scope creep!
• Changes are sized such that they do not touch excessive number of files.
• All future TODOs are captured in issues, which are referenced in code comments.
• These code changes follow the ScubaGear content style guide.
• Related issues these changes resolve are linked preferably via closing keywords.
• All relevant type-of-change labels added.
• All relevant project fields are set.
• All relevant repo and/or project documentation updated to reflect these changes.
• Unit tests added/updated to cover PowerShell and Rego changes.
• All relevant functional tests passed.
• All automated checks (e.g., linting, static analysis, unit/smoke tests) passed.

✅ Pre-merge checklist

• PR passed smoke test check.

• Feature branch has been rebased against changes from parent branch, as needed

  Use Rebase branch button below or use this reference to rebase from the command line.

• Resolved all merge conflicts on branch

• Notified merge coordinator that PR is ready for merge via comment mention

✅ Post-merge checklist

• Feature branch deleted after merge to clean up repository.
• Verified that all checks pass on parent branch (e.g., main or release-name) after merge.