sighs in powershell environment variables

Author: Vogel612

appveyor.yml

@@ -52,7 +52,7 @@ before_build:
   - nuget restore RubberduckMeta.sln
   - nuget restore Rubberduck.sln
 build_script:
-  - msbuild "C:\projects\rubberduck\Rubberduck.sln" -property:"Version=${APPVEYOR_BUILD_VERSION}" -verbosity:minimal -logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
+  - ps: msbuild "C:\projects\rubberduck\Rubberduck.sln" -property:"Version=$env:APPVEYOR_BUILD_VERSION" -verbosity:minimal -logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
 
 
 # TestScript depends on the build matrix to only contain Release cpu