Add missing backslashes to get_externals.bat (GH-5731)

PCbuild/get_externals.bat

@@ -65,7 +65,7 @@ for %%e in (%libraries%) do (
         git clone --depth 1 https://github.com/%ORG%/cpython-source-deps --branch %%e "%EXTERNALS_DIR%\%%e"
     ) else (
         echo.Fetching %%e...
-        %PYTHON% -E "%PCBUILD%get_external.py" -O %ORG% -e "%EXTERNALS_DIR%" %%e
+        %PYTHON% -E "%PCBUILD%\get_external.py" -O %ORG% -e "%EXTERNALS_DIR%" %%e
     )
 )
 
@@ -84,7 +84,7 @@ for %%b in (%binaries%) do (
         git clone --depth 1 https://github.com/%ORG%/cpython-bin-deps --branch %%b "%EXTERNALS_DIR%\%%b"
     ) else (
         echo.Fetching %%b...
-        %PYTHON% -E "%PCBUILD%get_external.py" -b -O %ORG% -e "%EXTERNALS_DIR%" %%b
+        %PYTHON% -E "%PCBUILD%\get_external.py" -b -O %ORG% -e "%EXTERNALS_DIR%" %%b
     )
 )