manibuilder: Add a version number to anita tags

NetBSD 7.1 mirrors have dropped its packages, so we need something
new and more flexibility. As we have always used NetBSD 7.1 so far,
prepend the tags with `7.1-`.

To avoid re-building old images, one can re-tag them, e.g.:

    $ docker image tag mani/anita:amd64 mani/anita:7.1-amd64

Change-Id: I44351805abab93c666d1d12c2bb17380caa75cec
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/51480
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
2 files changed