avoid reading policies from /etc/firefox on Linux

Georg noticed that Firefox 77 and ESR 68.9 will include a fix for this bug: "Read Linux policy from etc/opt directory" https://bugzilla.mozilla.org/show_bug.cgi?id=1469629

Here is the patch: https://hg.mozilla.org/releases/mozilla-esr68/rev/203a8c227a997c4ae7e970d0ec497d7292078d5c

Unfortunately, with that patch in place, policies will be read from /etc/firefox/policies/policies.json if it exists. For Tor Browser we do not want that behavior.

In the short run we can back out the Mozilla patch. In the long run should we handle this via a fixup for our #32418 (moved) patch.