bpo-34855: Fix EXTERNALS_DIR build variable for Windows (GH-11177)

.azure-pipelines/windows-appx-test.yml

@@ -36,7 +36,7 @@ jobs:
       Write-Host '##vso[task.setvariable variable=Py_IntDir]$(Build.BinariesDirectory)\obj'
       # UNDONE: Do not build to a different directory because of broken tests
       Write-Host '##vso[task.setvariable variable=Py_OutDir]$(Build.SourcesDirectory)\PCbuild'
-      Write-Host '##vso[task.setvariable variable=EXTERNAL_DIR]$(Build.BinariesDirectory)\externals'
+      Write-Host '##vso[task.setvariable variable=EXTERNALS_DIR]$(Build.BinariesDirectory)\externals'
     displayName: Update build locations
 
   - script: PCbuild\build.bat -e $(buildOpt)

.azure-pipelines/windows-steps.yml

@@ -8,7 +8,7 @@ steps:
     Write-Host '##vso[task.setvariable variable=Py_IntDir]$(Build.BinariesDirectory)\obj'
     # UNDONE: Do not build to a different directory because of broken tests
     Write-Host '##vso[task.setvariable variable=Py_OutDir]$(Build.SourcesDirectory)\PCbuild'
-    Write-Host '##vso[task.setvariable variable=EXTERNAL_DIR]$(Build.BinariesDirectory)\externals'
+    Write-Host '##vso[task.setvariable variable=EXTERNALS_DIR]$(Build.BinariesDirectory)\externals'
   displayName: Update build locations
 
 - script: PCbuild\build.bat -e $(buildOpt)

PCbuild/find_python.bat

@@ -24,7 +24,7 @@
 :begin_search
 @set PYTHON=
 
-@set _Py_EXTERNALS_DIR=%EXTERNAL_DIR%
+@set _Py_EXTERNALS_DIR=%EXTERNALS_DIR%
 @if "%_Py_EXTERNALS_DIR%"=="" (set _Py_EXTERNALS_DIR=%~dp0\..\externals)
 
 @rem If we have Python in externals, use that one