Add warning icon in the pipelines button in MRs

This MR makes the icon of a build in a pipeline to be in a third state (!) to symbolize that there were warnings during that build. This exit state does not cancel the build or mark it as failure it simply notifies the dev the there has been warnings.

Examples are shown here:

• Due to limitations of the Gitlab Powershell shell execute we cannot do this in our Windows build for the time being. This will be possible after we either update our CI machine to win10 or update its PS to PS7. In the meanwhile, windows warnings will fails the build.

Here is a pipeline displaying warnings: https://gitlab.kitware.com/vtk/vtk-m/-/pipelines/330878