[build] Use explicit posix.putenv() methods, etc.

We don't have the 'os' module in the release build; we have a fork of
the lower level 'posix' module in native/posixmodule.c.

Bad: I shouldn't have to manually edit posix_methods.def ...

build/oil-defs/Python-2.7.13/Modules/posixmodule.c/posix_methods.def

@@ -32,6 +32,7 @@ static PyMethodDef posix_methods[] = {
   {"fdopen", posix_fdopen, METH_VARARGS},
   {"isatty", posix_isatty, METH_VARARGS},
   {"pipe", posix_pipe, METH_NOARGS},
+  {"putenv", posix_putenv, METH_VARARGS},
   {"strerror", posix_strerror, METH_VARARGS},
 
   /* job control stuff */

osh/builtin_printf.py

@@ -14,7 +14,6 @@
 
 import sys
 import time
-import os
 
 from asdl import runtime
 from core import error
@@ -29,6 +28,8 @@
 from osh import string_ops
 from osh import word_compile
 
+import posix_ as posix
+
 from typing import Dict, List, TYPE_CHECKING, cast
 
 if TYPE_CHECKING:
@@ -335,9 +336,9 @@ def Run(self, cmd_val):
               tzcell = self.mem.GetCell('TZ')
               if tzcell and tzcell.exported and tzcell.val.tag_() == value_e.Str:
                 tzval = cast(value__Str, tzcell.val)
-                os.environ['TZ'] = tzval.s
-              elif 'TZ' in os.environ:
-                del os.environ['TZ']
+                posix.putenv('TZ', tzval.s)
+              elif posix.getenv('TZ'):
+                posix.unsetenv('TZ')
               time.tzset()
 
               # Handle special values: