Revert "Removed the vendored git-lfs and the GitLfs build flag"

This reverts commit efda811404.

Turns out that datalad is building git-annex against debian bullseye.
https://github.com/datalad/git-annex/issues/149

standalone/linux/stack-i386ancient.yaml

@@ -8,6 +8,7 @@ flags:
     dbus: false
     debuglocks: false
     benchmark: false
+    gitlfs: true
 packages:
 - '.'
 extra-deps: