Always reset timestamps in merge_zips

Jacoco uses ziptime and then merge_zips.  merge_zips is already rewriting
all the zip headers so its trivial to move the ziptime functionality
into merge_zips.

Bug: 308765940
Test: TestMergeZips
Change-Id: I8158c0b99878492233ac01fde3820e1951c9088e
diff --git a/cmd/merge_zips/merge_zips.go b/cmd/merge_zips/merge_zips.go
index a70a9d1..1aa6f6f 100644
--- a/cmd/merge_zips/merge_zips.go
+++ b/cmd/merge_zips/merge_zips.go
@@ -96,7 +96,9 @@
 	if err := ze.inputZip.Open(); err != nil {
 		return err
 	}
-	return zw.CopyFrom(ze.inputZip.Entries()[ze.index], dest)
+	entry := ze.inputZip.Entries()[ze.index]
+	entry.SetModTime(jar.DefaultTime)
+	return zw.CopyFrom(entry, dest)
 }
 
 // a ZipEntryFromBuffer is a ZipEntryContents that pulls its content from a []byte