Error downloading source with VTK 8.1.0 on Linux

When I updated the version to 8.1.0 and try to run the build script on Linux, I get the following error:

```
$ ./scripts/dockcross-manylinux-build-wheels.sh

[...]
[1/5] Performing update step for 'VTK-source-download'
FAILED: VTK-source-download-prefix/src/VTK-source-download-stamp/VTK-source-download-update
cd /work/standalone-x64-build/VTK-source && /usr/bin/cmake -P /work/standalone-x64-build/VTK-source-download-prefix/tmp/VTK-source-download-gitupdate.cmake
From https://github.com/kitware/VTK
dcf423b..c0f9780 master -> origin/master
dcf423b..ca2f34c nightly-master -> origin/nightly-master
Previous HEAD position was 782d7ed... Merge commit '6610577d' into release
HEAD is now at ab71574... Merge topic 'deprecate-hot' into release
Submodule 'VTK-m' (https://gitlab.kitware.com/vtk/vtk-m.git) registered for path 'ThirdParty/vtkm/vtk-m'
Cloning into 'ThirdParty/vtkm/vtk-m'...
warning: protocol restrictions not applied to curl redirects because
your curl version is too old (>= 7.19.4)
error: Unknown SSL protocol error in connection to gitlab.kitware.com:443 while accessing https://gitlab.kitware.com/vtk/vtk-m.git/info/refs?service=git-upload-pack
fatal: HTTP request failed
Clone of 'https://gitlab.kitware.com/vtk/vtk-m.git' into submodule path 'ThirdParty/vtkm/vtk-m' failed
CMake Error at /work/standalone-x64-build/VTK-source-download-prefix/tmp/VTK-source-download-gitupdate.cmake:157 (message):
Failed to update submodules in: '/work/standalone-x64-build/VTK-source/'
```
This doesn't seem right as the version of curl is recent enough. This seems like an intermittent error though and suddenly it seems to work again. Do you think it may be a good idea to build this without having to do a download each time, instead we could mount a local source directory and pass that into the script?

Start with scripts/dockcross-manylinux-build-wheels.sh and the CMake external-project update step shown in the log. Investigate the intermittent VTK-m submodule download failure and assess the proposed local source-directory or mounted-source workflow; done should be a documented, reliable build path that avoids the reported repeated download problem.

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