pack: use a specific mtime when packing (npm#20027)

> Thank god I found you.  Listen, can you meet me at Twin Pines Mall
> tonight at 1:15?  I've made a major breakthrough, I'll need your
> assistance.

Fixes: npm#19933
Fixes: npm#19968
PR-URL: npm#20027
Credit: @isaacs
Reviewed-By: @zkat

lib/pack.js

@@ -120,7 +120,9 @@ function packDirectory (mani, dir, target) {
         cwd: dir,
         prefix: 'package/',
         portable: true,
-        noMtime: true,
+        // Provide a specific date in the 1980s for the benefit of zip,
+        // which is confounded by files dated at the Unix epoch 0.
+        mtime: new Date('1985-10-26T08:15:00.000Z'),
         gzip: true
       }