fix(electron): add icon for AppImage build (#6257)

1. the icon is fixed in `/Applications`: https://github.com/toeverything/electron-forge-maker-appimage/commit/128b8c22f9b2e088ce4f88057749b0a8cd55ca98#diff-a694a3e854f53b066e34ec310e05bd18b4944c016455f6963f54a351784d5fa6L91
2. the App's icon MUST be 64x64 png and set via `setIcon`

![image](https://github.com/toeverything/AFFiNE/assets/584378/bbce0007-066b-413f-a85a-193acbbe5c13)

packages/frontend/electron/forge.config.mjs

@@ -10,6 +10,7 @@ import {
   buildType,
   icnsPath,
   iconUrl,
+  iconX64PngPath,
   icoPath,
   platform,
   productName,
@@ -77,6 +78,14 @@ const makers = [
   !process.env.SKIP_BUNDLE && {
     name: '@pengx17/electron-forge-maker-appimage',
     platforms: ['linux'],
+    config: {
+      icons: [
+        {
+          file: iconX64PngPath,
+          size: 64,
+        },
+      ],
+    },
   },
 ].filter(Boolean);