ci: publish releases to flakehub

.github/workflows/release.yml

@@ -4,11 +4,33 @@ on:
   push:
     tags:
       - v[0-9]+.[0-9]+.[0-9]+
+  workflow_dispatch:
+    inputs:
+      tag:
+        description: The existing tag to publish to FlakeHub
+        type: string
+        required: true
 
 jobs:
   release:
     runs-on: ubuntu-latest
+    if: github.event_name != 'workflow_dispatch'
     steps:
       - uses: softprops/action-gh-release@v1
         with:
           body: "[CHANGELOG.md](https://github.com/nix-community/nix-init/blob/main/CHANGELOG.md)"
+
+  flakehub:
+    runs-on: ubuntu-latest
+    permissions:
+      id-token: write
+      contents: read
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          ref: ${{ inputs.tag != null && format('refs/tags/{0}', inputs.tag) || '' }}
+      - uses: DeterminateSystems/nix-installer-action@v4
+      - uses: DeterminateSystems/flakehub-push@v3
+        with:
+          visibility: public
+          tag: ${{ inputs.tag }}