scripts/functions: always prefer the mirror

Currently, the mirror can be used either:
- as a fallback in case upstream is unavailable (default behavior)
- as the preferred source for downloads

But the most common use-case seems to provide a truely-LAN mirror
to speed up downloads in big corpos', and/or provide a 'trusted'
source for the tarballs.

So, make the following changes;
- if a mirror is specified, always try that before trying upstream

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

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