Include the build-id in firefox-l10n output name

While wokring on !665 (merged), I've noticed that we don't rebuild firefox-l10n if we change the list of supported language.

It shouldn't be a big deal, because I think as a matter of fact we rarely build the same commit with a different list of locales.

However, it should be pretty easy to fix, and even a comment with the list of the included locales should suffice, I think:

# We print the locales that are included to be sure to have a new build id if we change the list
# [% tmpl(c('var/locales').join(' ')) %]

EDIT: Turns out that we weren't using build-id for firefox-l10n.