stdenv-setup: Use `set -u` as much as possible

Older bash version, like those in the bootstrap tools and on macOS,
currently confuse variables defined as an empty array with undefined
variables. `${foo+"${foo[@]}"}` will prevent `set -u` problems with
empty arrays and older without making a single '' in the empty case.

Care is taken to `set +u` when running hooks so as to not break existing
packages.