reproducible FreeBSD: Actually ignore source tarball

The egrep argument was a glob, but egrep wants a(n extended) regex, so
this was not working correctly (GNU grep interprets this as matching
freebsd_main_git?.tar.xz, which is what's used in this case, whereas BSD
grep is more friendly and gives an error). But, in fact, the use of
egrep is redundant here, we can just get find to filter out the file in
the first place.

Note that because we're in the b1 directory, use find * and the file is
in the current directory we can use -regex despite it being for the
whole path and not the file name, as the two will be the same. Were this
to change, the regex would need appropriately prefixing to match a
directory prefix.

Signed-off-by: Holger Levsen <holger@layer-acht.org>