Review Mozilla 1960122: Use `MOZ_BUILD_DATE` in Fenix build configuration

🔍 Bugzilla Audit

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1960122

Description

This simplified our code for making Android reproducible, but maybe we can check it better, since it conflicted with the rebase (!1526 (merged), e6af7dc7)

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