Correct branch detection in build script

Author: nblumhardt

Build.ps1

@@ -4,7 +4,7 @@ if(Test-Path .\artifacts) { Remove-Item .\artifacts -Force -Recurse }
 
 & dotnet restore --no-cache
 
-$branch = $(git symbolic-ref --short -q HEAD)
+$branch = @{ $true = $env:APPVEYOR_REPO_BRANCH; $false = $(git symbolic-ref --short -q HEAD) }[$env:APPVEYOR_REPO_BRANCH -ne $NULL];
 $revision = @{ $true = $env:APPVEYOR_BUILD_NUMBER; $false = "local" }[$env:APPVEYOR_BUILD_NUMBER -ne $NULL];
 $suffix = @{ $true = ""; $false = "$branch-$revision"}[$branch -eq "master" -and $revision -ne "local"]