[2.7] Add missing backslashes in PCbuild bat files (GH-5056) (GH-5058)

(cherry picked from commit fe90efdc07ec70fd4ff28eac179448a41379cff1)

PCbuild/get_externals.bat

@@ -30,7 +30,7 @@ if "%DO_FETCH%"=="false" goto end
 :fetch
 
 if "%ORG%"=="" (set ORG=python)
-call "%PCBUILD%find_python.bat" "%PYTHON%"
+call "%PCBUILD%\find_python.bat" "%PYTHON%"
 
 git 2>&1 > nul
 if ERRORLEVEL 9009 (
@@ -61,7 +61,7 @@ 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% "%PCBUILD%get_external.py" -O %ORG% %%e
+        %PYTHON% "%PCBUILD%\get_external.py" -O %ORG% %%e
     )
 )
 
@@ -79,7 +79,7 @@ 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% "%PCBUILD%get_external.py" -b -O %ORG% %%b
+        %PYTHON% "%PCBUILD%\get_external.py" -b -O %ORG% %%b
     )
 )