scripts: Use proper PDK_ROOT when generating PDN
scripts: Use proper PDK_ROOT when generating PDN

This was "recreating" the path manually instead of using the configured
PDK_ROOT

Signed-off-by: Sylvain Munaut <349f318521b45672c77207abf1a5ed60eb011db1@246tNt.com>

efabless/openlane,efabless/openlane,efabless/openlane,efabless/openlane