Fix nimbus-fml reproducibility of 13.0a2-build1

We've hit a nimbus-fml reproducibility problem again, even though I thought we had fixed it by now (I was very lucky/unlucky and got several matching builds).

A way to reproduce the output outside the build system is:

./nimbus-fml generate --language kotlin --channel beta nimbus.fml.yaml output.kt

This can be run in a for loop, to check that now it produces different outputs sometimes, and that after the fix we should consistently get the same output.

Currently (115.2.0), it needs nimbus.fml.yaml, messaging.fml.yaml, onboarding.fml.yaml and possibly geckoview.fml.yaml (the latter should be needed only for channel release). They can all be found in the firefox-android repository.