build: derive PKG_SOURCE_DATE_EPOCH from the unpacked source tree

build: derive PKG_SOURCE_DATE_EPOCH from the unpacked source tree

Previously PKG_SOURCE_DATE_EPOCH came from a git log on the OpenWrt
package directory. That fails in shallow feed clones and in the SDK
(no .git), collapsing to the script's mtime and breaking .apk
reproducibility across SDK rebuilds and between buildbot and SDK.

With this comment, PKG_UNPACK generates a version.date file, later used to
determine a reproducible SOURCE_DATE_EPOCH. Since unpack happens after
download, the evaluation of SOURCE_DATE_EPOCH is now lazy, invoking the
`get_source_date_epoch.sh` script on every use.

While at it, drop export of PKG_SOURCE_DATE_EPOCH and clean it from the
ipkg-build script.

Link: https://github.com/openwrt/openwrt/issues/21579
Link: https://github.com/openwrt/openwrt/pull/21587
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Link: https://github.com/openwrt/openwrt/pull/23576
Signed-off-by: Paul Spooren <mail@aparcar.org>