Update purge workflow with manual trigger (radius-project#7622)

# Description

The purge test workflow is failing with authentication issues. The
documentation says to use GITHUB_TOKEN with package permissions but we
need to be able to test this first. This PR will update the manual
trigger so we can add permissions in a followup PR


https://docs.github.com/en/packages/learn-github-packages/about-permissions-for-github-packages#maintaining-access-to-packages-in-github-actions-workflows

https://docs.github.com/en/actions/publishing-packages/publishing-docker-images#publishing-images-to-github-packages

## Type of change

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- This pull request is a minor refactor, code cleanup, test improvement,
or other maintenance task and doesn't change the functionality of Radius
radius-project#7621 .

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Fixes: #issue_number

---------

Signed-off-by: sk593 <shruthikumar@microsoft.com>

.github/workflows/purge-test-resources.yaml

@@ -21,6 +21,8 @@ permissions:
   contents: read # Required for actions/checkout
 
 on:
+  # Enable manual trigger
+  workflow_dispatch:
   schedule:
     # Run twice a day
     - cron: "30 0,12 * * *"
@@ -33,7 +35,7 @@ env:
   ACTION_LINK: "${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}"
 
 jobs:
-  purge_gchr_dev:
+  purge_ghcr_dev:
     name: Delete old unused container images
     runs-on: ubuntu-latest
     if: github.repository == 'radius-project/radius'
@@ -101,9 +103,9 @@ jobs:
           done
   create_issue_on_failure:
     name: Create issue for failing purge test resources run
-    needs: [purge_gchr_dev, purge_azure_resources]
+    needs: [purge_ghcr_dev, purge_azure_resources]
     runs-on: ubuntu-latest
-    if: always() && (needs.purge_gchr_dev.result == 'failure' || needs.purge_azure_resources.result == 'failure')
+    if: always() && ${{ github.event_name != 'workflow_dispatch' }} && (needs.purge_ghcr_dev.result == 'failure' || needs.purge_azure_resources.result == 'failure')
     steps:
       - uses: actions/github-script@v7
         with: