workflows: fix package directory

[crasbe]

The suspected issue from #41 was not the cause for the PyPi failure. Instead, the output directory got changed in 2e698de#diff-ef2cef9f88b4fe09ca3082140e67f5ad34fb65fb6e228f119d3812261ae51449R54, which changed the path of the dist folder as well. The workflow was not changed, therefore the directory could not be found.

This also fixes a warning about the deprecated packages_dir variable.

[crasbe]

You can test this by running:

cbuec@W11nMate:~/RIOTstuff/riotctrl-kill/riotctrl$ tox -e build
* a lot of output *
cbuec@W11nMate:~/RIOTstuff/riotctrl-kill/riotctrl$ ls -l dist/
total 40
-rw-r--r-- 1 cbuec cbuec 17817 Apr 26 18:56 riotctrl-0.5.1-py3-none-any.whl
-rw-r--r-- 1 cbuec cbuec 20408 Apr 26 18:56 riotctrl-0.5.1.tar.gz