stdenv setup.sh: revert most of changes around #14907

I'm giving this up. Feel free to find some reasonable variant that works
at least on Linux and Darwin. Problems encountered:
- During bootstrap of Darwin stdenv `env -0` and some bash features
  don't work.
- Without `env -0` the contents of some multi-line phases is taken as
  variable declarations, which wouldn't typically matter, but the PR
  wanted to refuse bash-invalid names which would be occasionally
  triggered. This commit dowgrades that to a warning with explanation.