Make Powershell stop on errors

We had failing regression tests, but this went unnoticed because the goto-cl
regression tests were successful (which were the last common run in the cmdlet).

Author: tautschnig

buildspec-windows.yml

@@ -66,6 +66,9 @@ phases:
       - |
         $env:Path = "C:\tools\cygwin\bin;$env:Path"
         cmd /c 'call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" x64 && bash -c "make -C regression test BUILD_ENV=MSVC" '
+
+      - |
+        $env:Path = "C:\tools\cygwin\bin;$env:Path"
         cmd /c 'call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" x64 && bash -c "make -C regression/goto-cl test BUILD_ENV=MSVC" '
 
       - |