runtime: Windows arm/arm64 corefx build failures

Windows arm corefx builds are failing with:

04:14:29   D:\j\workspace\arm_cross_che---99792267\_\fx\src\Native\build-native.cmd arm Release Windows_NT
04:14:29   **********************************************************************
04:14:29   ** Visual Studio 2017 Developer Command Prompt v15.7.6
04:14:29   ** Copyright (c) 2017 Microsoft Corporation
04:14:29   **********************************************************************
04:14:31   **********************************************************************
04:14:31   ** Visual Studio 2017 Developer Command Prompt v15.7.6
04:14:31   ** Copyright (c) 2017 Microsoft Corporation
04:14:31   **********************************************************************
04:14:32   [vcvarsall.bat] Environment initialized for: 'x86_arm'
04:14:32   Commencing build of native components
04:14:32   
04:14:35   CMake is a pre-requisite to build this repository but it was not found on the path. Please install CMake from http://www.cmake.org/download/ and ensure it is on your path.
04:14:35   Failed to generate native component build project!

e.g.: https://ci.dot.net/job/dotnet_coreclr/job/master/job/jitstress/job/arm_cross_checked_windows_nt_corefx_baseline_bld/43/consoleFull

parent flow job: https://ci.dot.net/job/dotnet_coreclr/job/master/job/jitstress/job/arm_cross_checked_windows_nt_corefx_baseline_flow/

The build passes locally for me. I have cmake on my global path.

The failed job above was run on:

 Windows.10.Amd64.ClientRS4.DevEx.Open-1101034224124-0 (a0001BS)

Did the VMs change recently, esp. related to cmake installation?

@jashook @echesakovMSFT