`restore=$(shopt -p nullglob)` exits 1 when nullglob is off, which it is
in the workflow shell. Under the `bash -e` that GitHub runs steps with,
that ended "Pack artifact" before tar ran: every job on #512 and #550
built fine and then failed with nothing in the log but the command.
The self-test never saw it because `pack_packages ... || fail` suppresses
errexit. Enumerate package files with a loop instead of toggling shell
options, and add a test that calls both helpers under `bash -e` exactly
as the workflows do; it fails against the old helper.
Ryan Hughes
451280ace1