Fix early removal of rc-file for nix-shell

BASH_ENV causes all non-interactive shells called via eg. /etc/bashrc to
remove the rc-file before the main shell gets to run it. Completion
scripts will often do this. Fixes #976.

Adapted from and fixes #1034.

scripts/nix-build.in

@@ -288,7 +288,6 @@ foreach my $expr (@exprs) {
         writeFile(
             $rcfile,
             "rm -rf '$tmpDir'; " .
-            'unset BASH_ENV; ' .
             '[ -n "$PS1" ] && [ -e ~/.bashrc ] && source ~/.bashrc; ' .
             ($pure ? '' : 'p=$PATH; ' ) .
             'dontAddDisableDepTrack=1; ' .
@@ -302,7 +301,6 @@ foreach my $expr (@exprs) {
             'shopt -u nullglob; ' .
             'unset TZ; ' . (defined $ENV{'TZ'} ? "export TZ='${ENV{'TZ'}}'; " : '') .
             $envCommand);
-        $ENV{BASH_ENV} = $rcfile;
         my @args = ($ENV{NIX_BUILD_SHELL} // "bash");
         push @args, "--rcfile" if $interactive;
         push @args, $rcfile;