nixos/activation: fix activation script for non-POSIX shells (#46042)

This fixes an issue with shells like fish that are not fully POSIX
compliant. The syntax `ENV=val cmd' doesn't work properly in there.

This issue has been addressed in #45932 and #45945, however it has been
recommended to use a single shell (`stdenv.shell' which is either
`bash' or `sh') to significantly reduce the maintenance overload in the
future.

See https://github.com/NixOS/nixpkgs/issues/45897#issuecomment-417923464

Fixes #45897

/cc @FRidh @xaverdh @etu