Attempt to init cinder *only* when cinderx is enabled

Summary:
before, a few upstream tests fail (presumably because the *attempt* to init cinder causes "unexpected" (from the upstream test POV) additional imports to be done sooner):

```
third-party/python/3.12/facebook/bin/python3.12 -u -W default -bb -E -I -m test -m test -r -w -j 0 -u all,-audio,-gui,-network test_import test_threading test_site -v
```

{P1585526624}

fix it by commenting out the call to try to init cinder in `site.py` by default, and enabling at build-time (by editing `site.py` in the install genrule) only when cinderx is enabled.

note that we should probably change it again in the future to use a constraint instead of a macro arg, but that would be part of a bigger refactor to introduce a cinderx constraint.

Reviewed By: christycylee, thereversiblewheel

Differential Revision: D62604340

fbshipit-source-id: 118e11bdaddd2010c383c186d3ab5cdb7a741f76

Lib/site.py

@@ -644,7 +644,10 @@ def main():
     sethelper()
     if not sys.flags.isolated:
         enablerlcompleter()
-    init_cinder()
+    # Do not attempt to initialize cinder *by default*, as it's not relevant
+    # for regular MetaPython users (and cinderx is not even available).
+    # This gets enabled at build-time when cinderx is enabled (see D62604340).
+    # init_cinder()
     execsitecustomize()
     if ENABLE_USER_SITE:
         execusercustomize()