Build the buildapp binary in build/bin.
Don't leave empty files around when downloading fails.
Implement installing extensions from local cache.

The new GUC pginstall.serve_from_archive_dir allows the PostgreSQL
Extension Installer to directly use files found in pginstall.archive_dir
without asking the repository server about a possibly newer version of
the same archive file.

Note that it would be quite difficult to implement a smarter cache
management and go fetch a new version when we have one, because of the
way that the extension version strings have no other comparison operator
than equality.

So the current facility expects that only extension developers would set
pginstall.serve_from_archive_dir, and would then build a fresh version
of the archive file before trying to UPDATE or INSTALL again the extension.