Pin occt to novtk build variant

Description of work

Specify the novtk build of occt. Otherwise vtk is installed as an occt dependency. This then brings in a lot of other dependencies and adds a lot of unnecessary bloat. One of the packages that is installed is qt6-main, which has caused problems on some Windows machines when attempting to launch workbench from a Conda install (see attached issue). Removing the vtk dependency should resolve that issue.

Fixes #37581.

To test:

Once this build https://builds.mantidproject.org/job/build_packages_from_branch/823 has finished, the conda packages should be available on our conda channel. Create a new env and install with

mamba create -n novtk_test
mamba activate novtk_test
mamba install -c mantid/label/occt_novtk_test mantidworkbench

Verify that the novtk version of occt is installed in the conda environment (use the conda list to display all installed packages)
Also check that vtk and qt6-main are not in the environment.

This does not require release notes because it's a packaging change that won't directly affect users

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mantidproject / mantid