ci: rewrite SSH module URLs to HTTPS in terraform-validate workflow

In the GitHub Actions CI environment, the runner has no SSH key configured
for github.e.it.census.gov, so 'terraform init' fails when pulling modules
whose source URLs use the SSH git@ scheme (e.g. tags.tf, examples/simple/eks.tf).

Changes made to .github/workflows/terraform-validate.yaml:

1. Added 'Setup GITHUB Credentials' step (CSVD/gh-auth@main)
   - Obtains a short-lived GitHub App token using the repository's
     GH_APP_PEM_FILE secret and GH_APP_ID / GH_APP_INSTALLATION_ID vars.
   - This mirrors the credential setup already present in the release workflow.

2. Added 'Configure Git HTTPS for Terraform module pulls' step
   - Runs: git config --global url."https://oauth2:<token>@github.e.it.census.gov/".insteadOf "git@github.e.it.census.gov:"
   - This tells git to transparently rewrite any SSH git@ URL for this host
     to an authenticated HTTPS URL at the transport layer.
   - Terraform shells out to the system git binary for module fetches, so
     it honours this rewrite without any changes to .tf source URLs.

No .tf files are modified: SSH source URLs in tags.tf and examples/simple/eks.tf
remain intact so that developers using SSH keys locally are unaffected.

.github/workflows/terraform-validate.yaml

@@ -13,6 +13,18 @@ jobs:
       - name: Checkout code
         uses: CSVD/gh-actions-checkout@v4
 
+      - name: Setup GITHUB Credentials
+        id: github_credentials
+        uses: CSVD/gh-auth@main
+        with:
+          github_app_pem_file: ${{ secrets.GH_APP_PEM_FILE }}
+          github_app_installation_id: ${{ vars.GH_APP_INSTALLATION_ID }}
+          github_app_id: ${{ vars.GH_APP_ID }}
+
+      - name: Configure Git HTTPS for Terraform module pulls
+        run: |
+          git config --global url."https://oauth2:${{ steps.github_credentials.outputs.github_token }}@github.e.it.census.gov/".insteadOf "git@github.e.it.census.gov:"
+
       - name: Setup Terraform
         uses: CSVD/gh-actions-setup-terraform@v2
         with: