Make sure `toolkit.telemetry.enabled` is set to `false` in nightly builds
It's currently set to true
in https://searchfox.org/mozilla-central/rev/ce21a13035623c1d349980057d09000e70669802/modules/libpref/Preferences.cpp#3279 and locked in https://searchfox.org/mozilla-central/rev/ce21a13035623c1d349980057d09000e70669802/modules/libpref/Preferences.cpp#3311, thus avoiding our pref overrides.