build: patch Windows container, fixing Node 10 (#354)

Author: Benjamin E. Coe

.kokoro/test.bat

@@ -17,7 +17,15 @@
 cd /d %~dp0
 cd ..
 
-call npm install -g npm@latest || goto :error
+@rem The image we're currently running has a broken version of Node.js enabled
+@rem by nvm (v10.15.3), which has no npm bin. This hack uses the functional
+@rem Node v8.9.1 to install npm@latest, it then uses this version of npm to
+@rem install npm for v10.15.3.
+call nvm use v8.9.1 || goto :error
+call node C:\Users\kbuilder\AppData\Roaming\nvm-ps\versions\v8.9.1\node_modules\npm-bootstrap\bin\npm-cli.js i npm -g || goto :error
+call nvm use v10.15.3 || goto :error
+call node C:\Users\kbuilder\AppData\Roaming\nvm-ps\versions\v8.9.1\node_modules\npm\bin\npm-cli.js i npm -g || goto :error
+
 call npm install || goto :error
 call npm run test || goto :error