Added "isosub" to the list of colors not drawn by "plot pnm" in
magic for sky130, as it otherwise comes out deep purple.

sky130/magic/sky130.tech

diff --git a/sky130/magic/sky130.tech b/sky130/magic/sky130.tech
index 19fa806..15c63c5 100644
--- a/sky130/magic/sky130.tech
+++ b/sky130/magic/sky130.tech
@@ -6752,6 +6752,7 @@
      draw m3fill no_color_at_all
      draw m4fill no_color_at_all
      draw m5fill no_color_at_all
+     draw isosub no_color_at_all
      draw nwell cwell
 end