GitHub Actions build: fix version detection

Author: bkoelman

Diff for: .github/workflows/build.yml

@@ -92,7 +92,8 @@ jobs:
             $versionSuffix = $segments.Length -eq 1 ? '' : $segments[1..$($segments.Length - 1)] -join '-'
 
             [xml]$xml = Get-Content Directory.Build.props
-            $configuredVersionPrefix = $xml.Project.PropertyGroup[0].JsonApiDotNetCoreMongoDbVersionPrefix
+            $configuredVersionPrefix = $xml.Project.PropertyGroup.JsonApiDotNetCoreMongoDbVersionPrefix | Select-Object -First 1
+
             if ($configuredVersionPrefix -ne $versionPrefix) {
                 Write-Error "Version prefix from git release tag '$versionPrefix' does not match version prefix '$configuredVersionPrefix' stored in Directory.Build.props."
                 # To recover from this: