Fix long-running azure workflow syntax and re-enable for pull requests (

radius-project#6547)

# Description

This PR fixes the YAML syntax error in
https://github.com/radius-project/radius/actions/workflows/long-running-azure.yaml
and re-enables running the workflow on pull requests whenever the
workflow file is itself edited.

## Type of change

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- This pull request is a minor refactor, code cleanup, test improvement,
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Fixes: radius-project#6497

## Auto-generated summary

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### <samp>🤖 Generated by Copilot at 552ebbc</samp>

### Summary
🚀🧪🔒

<!--
1. 🚀 This emoji represents the pull_request trigger, which enables the
workflow to run on pull requests and launch the CI/CD pipeline.
2. 🧪 This emoji represents the testing and reviewing of the workflow
changes, which can help catch and fix any errors or bugs before merging.
3. 🔒 This emoji represents the security and protection of the main
branch, which can prevent unwanted or harmful changes from being
deployed.
-->
Enable pull request trigger for long-running Azure workflow. This allows
testing the `.github/workflows/long-running-azure.yaml` file changes
before merging.

> _`pull_request` runs_
> _workflow on branch changes_
> _autumn leaves falling_

### Walkthrough
* Enable workflow to run on pull requests that modify it
([link](https://github.com/radius-project/radius/pull/6547/files?diff=unified&w=0#diff-85cd86e7ae6da973e4808fe57f4fea7892ab78a499ba5aec72c0b3751a2cc226L44-R46))

Signed-off-by: Shubham Sharma <[email protected]>
Co-authored-by: Ryan Nowak <[email protected]>
Signed-off-by: willdavsmith <[email protected]>

.github/workflows/long-running-azure.yaml

@@ -41,9 +41,9 @@ on:
   schedule:
     # Run every 2 hours
     - cron: "0 */2 * * *"
-  # pull_request:
-  #   branches:
-  #     - main
+  pull_request:
+    branches:
+      - main
     paths:
       - '.github/workflows/long-running-azure.yaml'