Don't modify env vars for GSSAPI if network service is out of process

If the Network Service is running out of process, there's no reason to
set environment variables for GSSAPI. GSSAPI is only used from the
Network Service.
Setting environment variables is dangerous (see
https://crbug.com/1259918), so avoid it where we can.

Bug: 1258587
Change-Id: I43d7bc2b6259f4c4954df0d258cfb94c51c3db2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3223001
Reviewed-by: Felipe Andrade <fsandrade@chromium.org>
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#931692}