Patch libzip to return timestamps in the Unix epoch

We're not even using those timestamps, but doing the conversion to
local time takes a lot of time. For instance, this patch speeds up
'nix flake metadata nixpkgs` from 0.542s to 0.094s.

libzip-unix-time.patch

@@ -0,0 +1,19 @@
+commit 26e8c76ca84999fa5c0e46a9fc3aa7de80be2e9c
+Author: Eelco Dolstra <edolstra@gmail.com>
+Date:   Mon Oct 10 17:12:47 2022 +0200
+
+    Return time_t in the Unix epoch
+
+diff --git a/lib/zip_dirent.c b/lib/zip_dirent.c
+index 7fd2f7ce..5c050b4c 100644
+--- a/lib/zip_dirent.c
++++ b/lib/zip_dirent.c
+@@ -1018,7 +1018,7 @@ _zip_d2u_time(zip_uint16_t dtime, zip_uint16_t ddate) {
+     tm.tm_min = (dtime >> 5) & 63;
+     tm.tm_sec = (dtime << 1) & 62;
+ 
+-    return mktime(&tm);
++    return timegm(&tm);
+ }
+ 
+