When the SOURCE_DATE_EPOCH environment variable is set, use it to
override the timestamps of .ipk archive contents.
This ensures that .ipk archives built in environments without SCM
metadata (mainly the SDK) are reproducible between different runs.
Ref: https://github.com/openwrt/packages/issues/6954
Signed-off-by: Jo-Philipp Wich <jo@mein.io>
