Change script test for the modules symlink

What does this MR do?

The current -d test for the terraform modules symlink results in a situation where the symlink is never created - and will result in a confusing "Unreadable module directory" on first runs:

│ Error: Unreadable module directory
│
│ Unable to evaluate directory symlink: lstat ../../modules: no such file or directory
╵

The simplest correction for this is to change the test to ! -e and just create the symlink if an existing modules directory/symlink/file doesn't already exist.

This MR also removes an unused script_path variable.

Related issues

Closes #448 (closed)

Author's checklist

When ready for review, the Author applies the workflowready for review label and mention @gl-quality/get-maintainers:

• Merge request:
  • Corresponding Issue raised and reviewed by the GET maintainers team.
  • Merge Request Title and Description are up to date, accurate, and descriptive
  • MR targeting the appropriate branch
  • MR has a green pipeline
  • MR has no new security alerts in the widget from the Secret Detection and IaC Scan (SAST) jobs.
• Code:
  • Check the area changed works as expected. Consider testing it in different environment sizes (1k,3k,10k,etc.).
  • Documentation created/updated in the same MR.
  • If this MR adds an optional configuration - check that all permutations continue to work.
  • For Terraform changes: setup a previous version environment, then run a terraform plan with your new changes and ensure nothing will be destroyed. If anything will be destroyed and this can't be avoided please add a comment to the current MR.
• Create any follow-up issue(s) to support the new feature across other supported cloud providers or advanced configurations. Create 1 issue for each provider/configuration. Contact the Quality Enablement team if unsure.