ci: don't run gn debug build on older branches (#14584)

* ci: don't run gn debug build on older branches

Older branches that build using gyp do not run both a debug and testing build.

* Actually skip the build if debug

appveyor.yml

@@ -5,8 +5,10 @@ build_script:
     echo "Build worker image $env:APPVEYOR_BUILD_WORKER_IMAGE"
 
     &"C:\Program Files (x86)\Microsoft Visual Studio\Installer\vswhere.exe"
-
-    if(($env:APPVEYOR_PULL_REQUEST_HEAD_REPO_NAME -split "/")[0] -eq ($env:APPVEYOR_REPO_NAME -split "/")[0]) {
+    
+    if($env:SKIP_GYP_BUILD -eq "true") {
+      Write-warning "Skipping debug build for older branch"; Exit-AppveyorBuild
+    } elseif(($env:APPVEYOR_PULL_REQUEST_HEAD_REPO_NAME -split "/")[0] -eq ($env:APPVEYOR_REPO_NAME -split "/")[0]) {
       Write-warning "Skipping PR build for branch"; Exit-AppveyorBuild
     } else {
       Add-Path "$env:ProgramFiles (x86)\Windows Kits\10\Debuggers\x64"