Define COMMIT_NUMBER environment var (#782)

* Define COMMIT_NUMBER environment var to set the right FileVersion attribute on assembly files which was always 0.

* Keep COMMIT_NUMBER local var as a msbuild parameter given that GITHUB_ENV var are not seen by the step that defined the var.
It should be set a power shell var instead of a local var ( $Env:COMMIT_NUMBER = @($(git....)  or keep msbuild explicit parameter /p:COMMIT_NUMBER=$COMMIT_NUMBER

Author: claudiamurialdo

.github/workflows/Build.yml

@@ -96,6 +96,7 @@ jobs:
         Write-Output "Packge version to be used: $NUGET_PACKAGE_VERSION"
 
         echo "NUGET_PACKAGE_VERSION=$NUGET_PACKAGE_VERSION" >> $env:GITHUB_ENV
+        echo "COMMIT_NUMBER=$COMMIT_NUMBER" >> $env:GITHUB_ENV
         echo "IsPrerelease=$IsPrerelease" >> $env:GITHUB_ENV
 
         echo "::set-output name=NUGET_PACKAGE_VERSION::$NUGET_PACKAGE_VERSION"