nixos-manual: Fix stripping declaration prefixes

Regression introduced by e6cd147a

.

This broke all of the installer tests, because they needed to rebuild
the manual within the test machine, while it only has a closure of the
already pre-built system in place.

The problem here was just that the order of the arguments got mixed up
in stripAnyPrefixes, so it was actually trying to strip the path off the
prefix, not the other way around.

So in the end no prefix was stripped at all, so we ended up having full
store paths in the manual, which in turn caused the build within the VM
to fail, because the prefixes differed.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>