Use updated mirror for buster armhf, and update debian-installer

Use updated mirror for buster armhf, and update debian-installer

This reverts commit b838a9daeb3b ("production-config: Use a snapshot
for buster armhf")

Installation now fails with "Invalid Release signature", while
downloading the "Release" file of a repo.

But, using the main mirror for armhf boxes seems to work fine now.

To use the live mirror, we need to update the debian-installer as
there's a newer version of the kernel. Otherwise, installation fails
with the installer not able to find the disk.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>