bug#17150: Stale bootstrap/*/guile-2.0.9.tar.xz files are not detected

[Mark H Weaver]

I just realized that my x86_64 and Loongson 3A systems have spent an
enormous amount of time building the new guix master branch based on
outdated bootstrap/*/guile-2.0.9.tar.xz.

The issue is that if you simply "git pull" from a build directory with
older versions of bootstrap/*/guile-2.0.9.tar.xz, although the various
places where the hashes are stored are updated, those new hashes are
never checked against the existing files.  Therefore, you can proceed to
build an entire system based on an outdated bootstrap guile, and with
hashes that don't match what's on hydra and what other people are
building.

It would be good if the hashes were checked even if they are already
present in the build directory.

     Mark