Use the refs/follow/master/nightly ref for Nightly builds

[thewtex]

Use refs/follow/master/nightly in itk_common.cmake in the dashboard Git branch.

Once all nightly systems have transitioned to using this ref instead of nightly-master, we will not need nightly-master in the list of GitHub references.

https://gitlab.kitware.com/cmake/dashboard-scripts/blob/master/cmake_common.cmake#L238-243
https://gitlab.kitware.com/cmake/dashboard-scripts/blob/master/cmake_common.cmake#L139-154

[stale]

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