Add Flatpak bundle to the release workflow

Builds carrier-x86_64.flatpak from the linux job's already-built binary
rather than recompiling inside flatpak-builder's sandbox (which can't
see the shared cargo cache anyway, making an in-sandbox build pure
duplicate work). com.randogoth.carrier.release.json is a separate
manifest from the main one, which stays as-is for local
`devbox run package` dev-from-source builds.

com.randogoth.carrier.release.json

@@ -0,0 +1,45 @@
+{
+  "id": "com.randogoth.carrier",
+  "runtime": "org.gnome.Platform",
+  "runtime-version": "51",
+  "sdk": "org.gnome.Sdk",
+  "command": "carrier",
+  "finish-args": [
+    "--socket=wayland",
+    "--socket=fallback-x11",
+    "--device=dri",
+    "--share=network"
+  ],
+  "cleanup": [
+    "/share/doc"
+  ],
+  "modules": [
+    {
+      "name": "carrier",
+      "buildsystem": "simple",
+      "build-commands": [
+        "install -Dm755 carrier-linux-x86_64 /app/bin/carrier",
+        "install -Dm644 data/com.randogoth.carrier.desktop /app/share/applications/com.randogoth.carrier.desktop",
+        "install -Dm644 data/com.randogoth.carrier.metainfo.xml /app/share/metainfo/com.randogoth.carrier.metainfo.xml",
+        "install -Dm644 data/icons/com.randogoth.carrier.png /app/share/icons/hicolor/512x512/apps/com.randogoth.carrier.png",
+        "install -Dm644 data/icons/com.randogoth.carrier-256.png /app/share/icons/hicolor/256x256/apps/com.randogoth.carrier.png",
+        "install -Dm644 data/icons/com.randogoth.carrier-128.png /app/share/icons/hicolor/128x128/apps/com.randogoth.carrier.png"
+      ],
+      "sources": [
+        {
+          "type": "dir",
+          "path": ".",
+          "skip": [
+            "target",
+            ".git",
+            ".jj",
+            ".flatpak",
+            ".flatpak-builder",
+            ".flatpak-repo",
+            ".flatpak-build"
+          ]
+        }
+      ]
+    }
+  ]
+}