Add SOURCE_DATE_EPOCH for melange builds so that image has correct created timestamp

[jdolitsky]

Currently the images with melange builds (i.e. wolfictl) have a created timestamp matching the last built apk:

$ crane config ghcr.io/wolfi-dev/melange:latest@sha256:0cf85235ed98ad83f5d480210db8a8b3d513298d60aa5c56fd137920a69d4924 | jq -r .created
2023-06-06T21:13:45Z

we should set the SOURCE_DATE_EPOCH appropriately during the melange build so that this timestamp matches the last commit to the tool used (for example using git log -1 --pretty=%ct)