[copilot] Add externals directory caching (#1225)

## Summary

This PR adds caching for the `./externals` directory in the
`copilot-setup-steps.yml` workflow to improve build performance by
avoiding redundant Maven artifact downloads.

## Problem

The `dotnet cake` build process downloads hundreds of Maven artifacts
(AndroidX, Google Play Services, Firebase, etc.) to the `./externals`
directory during the binderator step. Without caching, these artifacts
are re-downloaded on every workflow run, significantly increasing
build time and network usage.

## Solution

Added explicit cache management using GitHub Actions cache:

- **Cache Key**: Based on `config.json` content hash (`externals-${{ hashFiles('config.json') }}`)
- **Cache Path**: `./externals` directory
- **Strategy**: Restore before `dotnet cake`, save after (only on cache miss)

## Implementation Details

```yaml
# Restore cache before build
- name: Restore externals cache
  uses: actions/cache/restore@v4
  with:
    path: ./externals
    key: externals-${{ hashFiles('config.json') }}

# Save cache after successful artifact download (only if cache miss)
- name: Save externals cache
  uses: actions/cache/save@v4
  if: steps.cache-externals-restore.outputs.cache-hit != 'true'
  with:
    path: ./externals
    key: externals-${{ hashFiles('config.json') }}
```

## Benefits

- **Faster builds**: Eliminates re-downloading of Maven artifacts when `config.json` hasn't changed
- **Reduced network usage**: Avoids unnecessary downloads of hundreds of Android library artifacts
- **Improved developer experience**: Faster feedback for workflow runs
- **Cost efficiency**: Reduces GitHub Actions compute time

## Cache Invalidation

The cache automatically invalidates when:
- `config.json` content changes (new/updated Maven artifacts)
- Manual cache clearing through GitHub UI
- 7-day GitHub Actions cache expiration

This ensures builds always use the correct artifacts while maximizing cache efficiency.

Co-authored-by: mattleibow <1096616+mattleibow@users.noreply.github.com>

Author: Copilot

.github/workflows/copilot-setup-steps.yml

@@ -27,10 +27,31 @@ jobs:
     - name: Restore dotnet tools
       run: dotnet tool restore
 
+    # Cache the externals directory based on config.json content
+    # This caches the Maven artifacts downloaded by the binderator tool
+    # to avoid re-downloading the same artifacts on subsequent runs
+    - name: Restore externals cache
+      id: cache-externals-restore
+      uses: actions/cache/restore@v4
+      with:
+        path: ./externals
+        key: externals-${{ hashFiles('config.json') }}
+
     - name: Run dotnet cake
       run: dotnet cake
       continue-on-error: true
 
+    # Save the externals cache after dotnet cake runs
+    # This ensures the cache reflects the original config.json state
+    # before any potential modifications by the build process
+    - name: Save externals cache
+      id: cache-externals-save
+      uses: actions/cache/save@v4
+      if: steps.cache-externals-restore.outputs.cache-hit != 'true'
+      with:
+        path: ./externals
+        key: externals-${{ hashFiles('config.json') }}
+
     - name: Display environment info
       run: |
         echo "=== Environment Summary ==="