ci: add the (Nix) flakestry workflow

.github/workflows/nix-flakestry.yml

@@ -0,0 +1,23 @@
+# Make the flake discoverable on https://flakestry.dev
+name: "Publish a flake to flakestry"
+on:
+    push:
+        tags:
+        - "v?[0-9]+.[0-9]+.[0-9]+"
+        - "v?[0-9]+.[0-9]+"
+    workflow_dispatch:
+        inputs:
+            tag:
+                description: "The existing tag to publish"
+                type: "string"
+                required: true
+jobs:
+    publish-flake:
+        runs-on: ubuntu-latest
+        permissions:
+            id-token: "write"
+            contents: "read"
+        steps:
+            - uses: flakestry/flakestry-publish@main
+              with:
+                version: "${{ inputs.tag || github.ref_name }}"