defaultUnpack: Preserve timestamps when copying files

Commit 6d928ab6 changed this to not
preserve timestamps. However, that results in non-determinism; in
particular, it gives us a broken $SOURCE_DATE_EPOCH (especially for
everything using fetchFromGitHub). Builds affected by timestamps <
1980 should be fixed in some other way (e.g. changing the timestamp to
some fixed date > 1980).