All changes in this patch were performed automatically with `shfmt` with
configuration flags specified in `.pre-commit-config.yaml`.
In addition to fixing whitespace the roundtrip through shfmt's AST also
transforms command substitutions
`cmd`
# becomes
$(cmd)
and some redirects
>&2 echo "msg"
# becomes
echo >&2 "msg"
This oversubscribes our cores 2x, which testing shows we actually
run with at times: speedup is around a third on average for builds,
and a bit more than that for testing.
Also some light Bashification in ci/build.sh, for consistency.
