nixos/install-grub: stop using bare file handles for readFile/WriteFile

}

sub readFile {

    my ($fn) = @_; local $/ = undef;

    open FILE, "<$fn" or return undef; my $s = <FILE>; close FILE;

    local $/ = "\n"; chomp $s; return $s;

    my ($fn) = @_;

    # enable slurp mode: read entire file in one go

    local $/ = undef;

    open my $fh, "<$fn" or return undef;

    my $s = <$fh>;

    close $fh;

    # disable slurp mode

    local $/ = "\n";

    chomp $s;

    return $s;

}

sub writeFile {

    my ($fn, $s) = @_;

    open FILE, ">$fn" or die "cannot create $fn: $!\n";

    print FILE $s or die;

    close FILE or die;

    open my $fh, ">$fn" or die "cannot create $fn: $!\n";

    print $fh $s or die "cannot write to $fn: $!\n";

    close $fh or die "cannot close $fn: $!\n";

}

sub runCommand {