Replace Linux zip build with tar.xz.

The filename of the Linux zip now conflicts with the macOS zip needed
by the auto-updater, and zip isn’t a usual format for Linux anyway.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

package.json

@@ -81,7 +81,7 @@
       "description": "Zulip Desktop Client for Linux",
       "target": [
         "deb",
-        "zip",
+        "tar.xz",
         "AppImage",
         "snap"
       ],