substituteAll: Enumerate environment variables more reliably

Getting the names of all environment variables is tricky. The previous
implementation easily got confused by multi-line variables. The new
one is more reliable but not still not perfect.

This works around a segfault in Bash 4.3, where the expression
"${!var}" (where var="-9") crashes under certain conditions.

http://hydra.nixos.org/build/16693445

pkgs/stdenv/generic/setup.sh

@@ -386,7 +386,7 @@ substituteAll() {
     local output="$2"
 
     # Select all environment variables that start with a lowercase character.
-    for envVar in $(env | sed "s/^[^a-z].*//" | sed "s/^\([^=]*\)=.*/\1/"); do
+    for envVar in $(env | sed -e $'s/^\([a-z][^=]*\)=.*/\\1/; t \n d'); do
         if [ "$NIX_DEBUG" = "1" ]; then
             echo "$envVar -> ${!envVar}"
         fi