ENH: update Shark version for var importance estimation bug

A small bug in Shark has been corrected. It does not impact OTB, since it was related to variable importance estimation and we don't use that, but we may in the future. But I don't know if this should be done for 6.6 of if we wait until after the release.

We just need to update External_shark.cmake with: URL "https://github.com/Shark-ML/Shark/archive/67990bcd2c4a90a27be97d933b3740931e9da141.zip" URL_MD5 9ad7480a4f9832b63086b9a683566187

The only difference between the current version in the Superbuild and this changeset is the correction of the bug.