buildout4.cfg: Use whatever setuptools/pip versions are already avail…

…able.

Otherwise you get a buildout restart, and then it can't find the buildout file anymore.
Sample error on gha:

```
Getting distribution for 'pip==20.3.4'.
Got pip 20.3.4.
Upgraded:
  pip version 20.3.4;
Restarting.
Generated script '/__w/Products.CMFCore/Products.CMFCore/.tox/py27/bin/buildout'.
While:
  Initializing.
Error: Couldn't open /__w/Products.CMFCore/Products.CMFCore/.tox/py27/buildout4.cfg
```

buildout4.cfg

@@ -5,3 +5,8 @@ extends =
 
 [versions]
 Products.CMFCore =
+# Use whatever setuptools/pip versions are already available.
+# Otherwise you get a buildout restart, and then it can't find
+# the buildout file anymore.
+pip =
+setuptools =