ci: Create flakehub-publish-tagged.yml

This will get `std` listed on https://flakehub.com/flakes

.github/workflows/flakehub-publish-tagged.yml

@@ -0,0 +1,17 @@
+on:
+  push:
+    tags:
+      - "v*.*.*"
+jobs:
+  publish:
+    runs-on: "ubuntu-latest"
+    permissions:
+      id-token: "write"
+      contents: "read"
+    steps:
+      - uses: "actions/checkout@v3"
+      - uses: "DeterminateSystems/nix-installer-action@main"
+      - uses: "DeterminateSystems/flakehub-push@main"
+        with:
+          visibility: "public"
+          name: "divnix/std"