Fix calling tpchgen-cli in .venv

[EnricoMi]

The tpchgen-cli binary is installed in .venv/bin, but it is invoked without the path. This only works if .venv is activated.

$ SCALE_FACTOR=1.0 make run-polars
...
mkdir -p "data/tables/scale-1.0"
tpchgen-cli --output-dir="data/tables/scale-1.0" --format=tbl -s 1.0
/bin/bash: Zeile 1: tpchgen-cli: Befehl nicht gefunden
make: *** [Makefile:53: data/tables/.generated] Fehler 127

Using $(VENV_BIN)/tpchgen-cli in the Makefile makes this robust and work without the virtual env activated.