Don't set $PS1 in non-interactive shells

Shouldn't really matter, but you never know.

scripts/nix-build.in

@@ -209,7 +209,7 @@ foreach my $expr (@exprs) {
             '[ -e $stdenv/setup ] && source $stdenv/setup; ' .
             ($pure ? '' : 'PATH=$PATH:$p; ') .
             'set +e; ' .
-            'PS1="\n\[\033[1;32m\][nix-shell:\w]$\[\033[0m\] "; ' .
+            '[ -n "$PS1" ] && PS1="\n\[\033[1;32m\][nix-shell:\w]$\[\033[0m\] "; ' .
             'unset NIX_ENFORCE_PURITY; ' .
             'shopt -u nullglob; ' .
             $envCommand);