This should output something with PV, but it doesn't.

uses_pv.nix

with import <pkgs>;

stdenv.mkDerivation {
  name = "test_of_stuff";
  buildInputs = [python pv];
  buildCommand = ''
    echo "doing some random stuff..."
    cat <<EOF | python
    import random, string, os
    letters = string.letters + ' '
    f = open(os.path.join(os.environ['TMPDIR'], 'data.dat'), 'w')
    for i in range(100000):
        f.write(random.choice(letters))
    EOF
    _size="$(du -sk $TMPDIR/data.dat | awk '{s+=$1} END {print s}')k"
    tar -cf - $TMPDIR/data.dat | pv -pte -s $_size | gzip > $TMPDIR/data.tar.gz
    ls -l $TMPDIR/data.*
    echo "done"
  '';
}