defaultUnpack: Preserve timestamps when copying files

Commit 6d928ab684327e0eeb1bf6cd889d57ca7127e8a7 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).

pkgs/stdenv/generic/setup.sh

@@ -480,9 +480,11 @@ _defaultUnpack() {
     if [ -d "$fn" ]; then
 
         stripHash "$fn"
-        # We can't preserve hardlinks because they may have been introduced by
-        # store optimization, which might break things in the build
-        cp -pr --reflink=auto --no-preserve=timestamps "$fn" $strippedName
+
+        # We can't preserve hardlinks because they may have been
+        # introduced by store optimization, which might break things
+        # in the build.
+        cp -pr --reflink=auto "$fn" $strippedName
 
     else