ci(vercel): add Content-Disposition header on installer

This allows doing something like

  curl -O https://install.ohmyz.sh

and have the `install.sh` file automatically saved to its right name.

.github/workflows/installer/vercel.json

@@ -2,7 +2,16 @@
   "headers": [
     {
       "source": "/((?!favicon.ico).*)",
-      "headers": [{ "key": "Content-Type", "value": "text/plain" }]
+      "headers": [
+        {
+          "key": "Content-Type",
+          "value": "text/plain"
+        },
+        {
+          "key": "Content-Disposition",
+          "value": "inline; filename=\"install.sh\""
+        }
+      ]
     }
   ],
   "rewrites": [