environment: copy `PLATFORM` to `NW_PLATFORM`

appveyor.yml

@@ -18,6 +18,9 @@ install:
   - ps: Update-NodeJsInstallation $env:nodejs_version $env:Platform
   - node --version
   - npm --version
+  - set nw_platform=%PLATFORM%
+  - echo %PLATFORM%
+  - echo %NW_PLATFORM%
   - SET PATH=c:\python27;%PATH%
   - SET PATH=C:\Program Files (x86)\MSBuild\12.0\bin\;%PATH%
   # add local node-pre-gyp dir to path
@@ -30,7 +33,7 @@ install:
   # make commit message env var shorter
   - SET CM=%APPVEYOR_REPO_COMMIT_MESSAGE%
   - if not "%CM%" == "%CM:[publish binary]=%" node-pre-gyp --msvs_version=2013 publish
-  - call scripts\build_for_node_webkit.cmd %PLATFORM%
+  - call scripts\build_for_node_webkit.cmd %NW_PLATFORM%
 
 build: OFF
 test: OFF