[icudotnet] Change node label
[icudotnet] Change node label

Don't hardcode machine name but instead use labels to describe the
desired build agent(s). This makes it easier to swap out machines.

Change-Id: Ib04f07bd06b397483069946f9ab7745fae97f780

sillsdev/ci-builder-scripts,sillsdev/ci-builder-scripts,sillsdev/ci-builder-scripts