abuild: Fix abuild rootbld
Without this change abuild rootbld would fail with
touch: invalid date '@'
Because SOURCE_DATE_EPOCH wasn't set when abuild rootbld was used. This is a bug introduced in 71d9d523. Instead of reverting the aforementioned commit move the SOURCE_DATE_EPOCH initialization to a custom function and also call it from the abuild rootbld function.
Fixes #9978 (closed)