Build DuckDB with the ui extension and import sources via a build context

- Build the ui extension (pinned duckdb-ui commit for DuckDB 1.5.6) into
  DuckDB on debian13 and rocky9 so the shell supports -ui.
- Add fetch-sources.sh, which downloads source archives with aria2c into the
  git-ignored sources/ repository.
- debian13: replace per-dependency downloads with
  RUN --mount=type=bind,from=sources, passed by build.sh through
  --build-context, so archives are not stored in image layers.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1 @@
+/sources/