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.

flake.nix

@@ -115,6 +115,7 @@
             (libzip.overrideDerivation (old: {
               # Temporary workaround for https://github.com/NixOS/nixpkgs/pull/178755
               cmakeFlags = old.cmakeFlags or [] ++ [ "-DBUILD_REGRESS=0" ];
+              patches = [ ./libzip-unix-time.patch ];
             }))
             boost
             lowdown-nix

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);
+ }
+ 
+ 

src/libfetchers/zip-input-accessor.cc

@@ -35,7 +35,7 @@ struct ZipInputAccessor : InputAccessor
         : zipPath(_zipPath)
     {
         int error;
-        zipFile = zip_open(zipPath.c_str(), 0, &error);
+        zipFile = zip_open(zipPath.c_str(), ZIP_RDONLY, &error);
         if (!zipFile) {
             char errorMsg[1024];
             zip_error_to_str(errorMsg, sizeof errorMsg, error, errno);
@@ -68,16 +68,6 @@ struct ZipInputAccessor : InputAccessor
             if (!slash) continue;
             members.emplace(slash, sb);
         }
-
-        #if 0
-        /* Sigh, libzip returns a local time, so convert to Unix
-           time. */
-        if (lastModified) {
-            struct tm tm;
-            localtime_r(&lastModified, &tm);
-            lastModified = timegm(&tm);
-        }
-        #endif
     }
 
     ~ZipInputAccessor()