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] 62fc8859c1
[2] 81df035429
a09d9e7cd4