Simple modification to build-namelist to remove the need for the -lnd_root argument

#3570 adds a new argument to build-namelist. There's a simple change that can be done to remove the need for that argument. This helps with #1763 where we want to deprecate command line arguments in favor of using env XML variables. Which also helps to move us towards being able to replace build-namelist with python buildnml extended to cover what build-namelist does.

What I'm thinking of is the top of the perl module gives you $ProgDir, which is basically "$landroot/bld". So we can get it with:

``` perl
my $landroot = abs_path("$ProgDir/..");
```

I think this is just a little simpler and doesn't require that cross communication bit.

_Originally posted by @ekluzek in https://github.com/ESCOMP/CTSM/pull/3570#discussion_r2624972979_