[bug] [test] Fix patch_os_environ_helper (#6017)

Related PR = #6002
In #6002, `test_init_arg` and `test_init_arch` fails on Windows. The
failure is caused by reading outdated offline cache format. However, we
have set offline cache to off and set the offline cache directory to a
new directory in the environment variables in `run_tests.py`, so this
should not happen. Then, I discovered that function
`patch_os_environ_helper` which intends to remove only a few specified
environment variables (`excludes`) accidentally removes all the
environment variables, and replaced them with `custom_environ`. This PR
fixes the bug and lets it work as expected.

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tests/python/test_runtime.py

@@ -41,7 +41,7 @@ def patch_os_environ_helper(custom_environ: dict, excludes: dict):
         environ[key] = custom_environ[key]
     try:
         cached_environ = os.environ
-        os.environ = custom_environ
+        os.environ = environ
         yield os.environ
     finally:
         os.environ = cached_environ