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

(cherry picked from commit f924a1605569d81d38026fdf434e9af59543f56c)

appveyor.yml

@@ -2,7 +2,9 @@ build_cloud: electron-16
 image: electron-16-vs2015
 build_script:
 - ps: >-
-    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"
@@ -64,4 +66,4 @@ deploy_script:
         Write-Output "Uploading Electron release distribution to github releases"
         & python script\upload.py
       }
-    }
+    }