Create universal x86-64/arm64 mac builds

In #40158 (closed) we add support for macOS aarch64 builds. However, to avoid doubling the space used by macOS builds we should create universal x86-64/arm64 dmg files.

The tickets on mozilla side are:

https://bugzilla.mozilla.org/show_bug.cgi?id=1675740
https://bugzilla.mozilla.org/show_bug.cgi?id=1675384

I think the scripts from Mozilla we can use for that are:

taskcluster/scripts/misc/unify.sh
toolkit/mozapps/installer/unify.py

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