Do not rely on $stdenv/setup to set output variables

Instead of relying on setup script to set output variables when
structured attributes are enabled, iterate over the values of an
outputs associative array.

See also
374fa3532e/pkgs/stdenv/generic/setup.sh (L23-L26)

tests/functional/shell.nix

@@ -21,14 +21,6 @@ let pkgs = rec {
       export PATH=$PATH:$pkg/bin
     done
 
-    # mimic behavior of stdenv for `$out` etc. for structured attrs.
-    if [ -n "''${NIX_ATTRS_SH_FILE}" ]; then
-      for o in "''${!outputs[@]}"; do
-        eval "''${o}=''${outputs[$o]}"
-        export "''${o}"
-      done
-    fi
-
     declare -a arr1=(1 2 "3 4" 5)
     declare -a arr2=(x $'\n' $'x\ny')
     fun() {