patch-shebangs: use --build for auto patch shebangs

In strictDeps=false, autoPatchshebangs should use
--build (corresponding to PATH) to lookup commands. This restores the
previous behavior of patchshebangs so that we don’t break stuff that
isn’t careful in the buildInputs vs. nativeBuildInputs distinction.
Unfortunately this won’t work under cross compilation.

pkgs/stdenv/generic/setup.sh

@@ -257,17 +257,9 @@ shopt -s nullglob
 
 # Set up the initial path.
 PATH=
-HOST_PATH=
 for i in $initialPath; do
     if [ "$i" = / ]; then i=; fi
     addToSearchPath PATH "$i/bin"
-
-    # For backward compatibility, we add initial path to HOST_PATH so
-    # it can be used in auto patch-shebangs. Unfortunately this will
-    # not work with cross compilation.
-    if [ -z "${strictDeps-}" ]; then
-        addToSearchPath HOST_PATH "$i/bin"
-    fi
 done
 
 if (( "${NIX_DEBUG:-0}" >= 1 )); then