build: fixup version.date creation for source archives

build: fixup version.date creation for source archives

Published sources archives may contain a mixture of MTIMEs, so taking the time
of the first file found varies based on filesystem order. To fix this, sort all
MTIMEs and take the latest.

Since piping into a file directly creates that file, we also need to
specifically tell `find` to ignore `version.date`.

Furthermore, move this prior to the ./src copy step, since for some packages,
the OpenWrt build system ships extra ./src files, which would introduce the
wrong timestamps again.

Fix: e36c2946b7b build: derive PKG_SOURCE_DATE_EPOCH from the unpacked source tree
Link: https://github.com/openwrt/openwrt/pull/23756
Signed-off-by: Paul Spooren <mail@aparcar.org>