[CI] Locate vcomp140.dll from System32 directory

[hcho3]

After I re-built Jenkins testing pipeline from scratch, I found out that I cannot always expect the path C:\\Program Files (x86)\\Microsoft Visual Studio\\Shared\\14.0\\VC\\redist\\x64\\Microsoft.VC140.OpenMP\\vcomp140.dll to exist. Since vcomp140.dll is a system lib, it's better to use C:\\Windows\\System32\\vcomp140.dll

[trivialfis]

Windows is really out of my reach, maybe @RAMitchell can help here . :-)

[hcho3]

I found the reason why testxgboost.exe was crashing. For each Windows worker, I had to install a system lib called Microsoft Visual C++ Redistributable for Visual Studio, so that the worker can run testxgboost.exe.