[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failure is caused by reading outdated offline cache format.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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