Update appveyor.yml

appveyor.yml

@@ -2,6 +2,17 @@ version: 4.0.{build}
 skip_tags: true
 image: Visual Studio 2022
 
+install:
+  # If a newer build is queued for the same PR, cancel this one.
+  # The AppVeyor 'rollout builds' option is supposed to serve the same
+  # purpose, but it is problematic because it tends to cancel builds pushed
+  # directly to main/master instead of just PR builds (or the converse).
+  # credits: JuliaLang developers.
+  - ps: if ($env:APPVEYOR_PULL_REQUEST_NUMBER -and $env:APPVEYOR_BUILD_NUMBER -ne ((Invoke-RestMethod `
+          https://ci.appveyor.com/api/projects/$env:APPVEYOR_ACCOUNT_NAME/$env:APPVEYOR_PROJECT_SLUG/history?recordsNumber=50).builds | `
+          Where-Object pullRequestId -eq $env:APPVEYOR_PULL_REQUEST_NUMBER)[0].buildNumber) { `
+            throw "There are newer queued builds for this pull request, failing early." }  
+
 build_script:
 - cmd: nuget pack MSBuild.Version.Target.nuspec -Version %APPVEYOR_BUILD_VERSION%