Add weekly build workflow to ensure regular builds even during quiet periods (#226)

This PR adds a new GitHub Actions workflow that runs builds
automatically once per week, ensuring that builds are executed regularly
even when there are no recent changes to the main branch.

## Changes Made

- **New workflow file**: `.github/workflows/weekly-build.yml`
- **Schedule**: Runs every Sunday at midnight UTC using cron expression
`'0 0 * * 0'`
- **Manual trigger**: Includes `workflow_dispatch` for testing purposes
- **Build matrix**: Uses identical configuration to the existing
`build.yml` workflow
- **Artifact naming**: Weekly build artifacts are suffixed with
`-weekly` to distinguish them from push-triggered builds

## Rationale

The existing `build.yml` workflow only runs when code is pushed to main
or release branches. During periods of low activity, this could mean
builds don't run for extended periods, potentially missing issues with
dependencies, toolchain updates, or other environmental changes.

This weekly build ensures:
- Regular validation of the build process
- Early detection of dependency or environment issues
- Consistent artifact generation for downstream processes
- Maintained build confidence during quiet development periods

## Testing

The workflow can be manually triggered via the GitHub Actions UI for
immediate testing. The scheduled execution will begin automatically
according to the cron schedule.

Fixes #225.

<!-- START COPILOT CODING AGENT TIPS -->
---

💡 You can make Copilot smarter by setting up custom instructions,
customizing its development environment and configuring Model Context
Protocol (MCP) servers. Learn more [Copilot coding agent
tips](https://gh.io/copilot-coding-agent-tips) in the docs.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: eleanorjboyd <26030610+eleanorjboyd@users.noreply.github.com>

Author: Copilot

azure-pipelines/pre-release.yml

@@ -1,8 +1,15 @@
-# Run on a schedule
+# Run on both a schedule and weekly
 trigger:
   branches:
     include:
       - main
+schedules:
+  - cron: "0 0 * * 0"
+    displayName: Weekly build
+    branches:
+      include:
+        - main
+    always: true
 pr: none
 
 resources: