stdenv-setup: fix substituteAll with set -eu

Environment variable filter in substituteAll was not precise and produced
undefined and invalid variable names.  Vladimír Čunát tried to fix that in [1],
but `env -0` did not work during Darwin bootstrap, so [2] reverted this change
and replaced an error due to invalid variables with a warning.  Recently in #28057
John Ericson added `set -u` to `setup.sh` and undefined variables made the setup
fail during e.g. `nix-build -A gnat` with `setup: line 519: !varName: unbound
variable`.

[1] https://github.com/NixOS/nixpkgs/commit/62fc8859c10dd18b005b9bcaa0b429103d7661d9
[2] https://github.com/NixOS/nixpkgs/commit/81df0354290389128077e00edfd2368eeeea0c24