scripts/ipkg-build: fix a build regression introduced in r46360 (fixes #20082)

go to the $TOPDIR directory before trying to extract a git/svn timestamp

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46383 3c298f89-4303-0410-b956-a3cf2f4a3e73