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

(cherry picked from commit 6aedfa6b9ac324587f64133c23757a66a8f355bb)

Co-authored-by: antektek <45912913+antektek@users.noreply.github.com>
diff --git a/PCbuild/find_python.bat b/PCbuild/find_python.bat
index d0778dd..d5c787f 100644
--- a/PCbuild/find_python.bat
+++ b/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