STYLE: Use pinned version for source dir removal and fix ccache comment

- Use ExternalDataVersion variable instead of broad InsightToolkit-*
  glob for source directory removal.
- Fix misleading ccache --cleanup comment: it trims the cache to stay
  within CCACHE_MAXSIZE, not "flush to persistent cache dir".

Addresses review comments from @greptile-apps on PR #6011.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hjmjohnson

.github/workflows/pixi.yml

@@ -145,8 +145,8 @@ jobs:
             # Remove downloaded data tarballs (already extracted)
             rm -f InsightData-*.tar.gz
             # Remove extracted source tarball directory
-            rm -rf InsightToolkit-*
-            # Flush ccache to persistent cache dir and clear temp
+            rm -rf InsightToolkit-${{ env.ExternalDataVersion }}
+            # Trim ccache to stay within CCACHE_MAXSIZE and remove orphaned entries
             ccache --cleanup 2>/dev/null || true
             echo "****** df -h /  -- post cleanup"
             df -h /