Add Linux support to build scripts.

[surban]

• Port UpdateTemplate.cmd to Powershell as UpdateTemplate.ps1.
• Make build.ps1 compatible with PowerShell Core on Linux.

[vicancy]

Build failure:

[11:58:15][Step 3/3] [11:58:18] Finished 'clean' after 8.33 ms
[11:58:15][Step 3/3] [11:58:18] Starting 'build'...
[11:58:16][Step 3/3] Downloading NuGet.exe...
[11:58:16][Step 3/3]
[11:58:16][Step 3/3]
[11:58:16][Step 3/3] Directory: C:\Users\build\AppData\Local
[11:58:16][Step 3/3]
[11:58:16][Step 3/3]
[11:58:16][Step 3/3] Mode LastWriteTime Length Name
[11:58:16][Step 3/3] ---- ------------- ------ ----
[11:58:16][Step 3/3] d---- 2/26/2016 3:00 AM Nuget
[11:58:16][Step 3/3] Error : Update template
[11:58:16][Step 3/3]
[11:58:16][Step 3/3]
[11:58:16][Step 3/3]
[11:58:16][Step 3/3] [11:58:19] 'build' errored after 1.42 s
[11:58:16][Step 3/3] [11:58:19] Error: powershell ./build.ps1 -prod failed with code 1
[11:58:16][Step 3/3] at ChildProcess. (C:\BuildAgent\work\130feb8a19fc2a14\docfx\tools\Deployment\node_modules\child-process-promise\lib\index.js:132:23)
[11:58:16][Step 3/3] at emitTwo (events.js:106:13)
[11:58:16][Step 3/3] at ChildProcess.emit (events.js:194:7)
[11:58:17][Step 3/3] at ChildProcess.cp.emit (C:\BuildAgent\work\130feb8a19fc2a14\docfx\tools\Deployment\node_modules\cross-spawn\lib\enoent.js:40:29)
[11:58:17][Step 3/3] at maybeClose (internal/child_process.js:899:16)
[11:58:17][Step 3/3] at Process.ChildProcess._handle.onexit (internal/child_process.js:226:5)

[surban]

Building on Linux works, but some tests fail, probably due to hard-coded directory separators.

NuGet pack fails due to NuGet/Home#1946.

I guess the next step would be to add a build agent running on Linux?