Added Houdini 20 CI for old and new CXX11 ABI

[Idclip]

Here's an example of the Houdini CI run once this is merged and the weekly job udpates the cache: https://github.com/Idclip/openvdb/actions/runs/7113714217

@jmlait @e4lam We seem to have some issues building against Houdini 20 with clang. Seems to build fine with GCC:

 In file included from /__w/openvdb/openvdb/openvdb_houdini/openvdb_houdini/GeometryUtil.cc:8:
/__w/openvdb/openvdb/openvdb_houdini/openvdb_houdini/GeometryUtil.h:251:57: error: member access into incomplete type 'const GEO_Primitive'
            normal = mRefGeo.getGEOPrimitive(primOffset)->computeNormal();
                                                        ^
/github/home/houdini_install/hou/toolkit/include/GEO/GEO_PrimList.h:18:7: note: forward declaration of 'GEO_Primitive'
class GEO_Primitive;
      ^
In file included from /__w/openvdb/openvdb/openvdb_houdini/openvdb_houdini/GeometryUtil.cc:8:
/__w/openvdb/openvdb/openvdb_houdini/openvdb_houdini/GeometryUtil.h:257:63: error: member access into incomplete type 'const GEO_Primitive'
                    tmpN = mRefGeo.getGEOPrimitive(primOffset)->computeNormal();
                                                              ^
/github/home/houdini_install/hou/toolkit/include/GEO/GEO_PrimList.h:18:7: note: forward declaration of 'GEO_Primitive'
class GEO_Primitive;
      ^
2 errors generated.
gmake[2]: *** [openvdb_houdini/openvdb_houdini/CMakeFiles/openvdb_houdini.dir/build.make:121: openvdb_houdini/openvdb_houdini/CMakeFiles/openvdb_houdini.dir/GeometryUtil.cc.o] Error 1

[e4lam]

@Idclip Ah my bad, I had fixed this but didn't realized that I had touched openvdb_houdini headers because it was all over the code base. So I never triggered a PR for this:

--- dev/src/houdini/custom/DWA/vdb/GeometryUtil.h   2023/09/09 23:36:41 453698
+++ dev/src/houdini/custom/DWA/vdb/GeometryUtil.h   2023/09/09 23:39:09 453699
@@ -15,6 +15,7 @@
 #include <openvdb/util/Util.h> // for openvdb::util::COORD_OFFSETS

 #include <GU/GU_Detail.h>
+#include <GEO/GEO_Primitive.h>

 #include <algorithm> // for std::max/min()
 #include <memory>

EDIT: PS. Sorry for the slow reply as I've been on vacation.

[Idclip]

Thanks @e4lam! No worries, I didn't want to blindly add the include in case there was a deeper issue between clang/gcc, but this does indeed fix the clang build.

I've made this change, example workflow result here: https://github.com/Idclip/openvdb/actions/runs/7177898971

This should be good to go