ci(nix): refactor publish workflow

- `flakehub-backfill.yaml` and `flakehub-publish-tagged.yml` replaced by `publish-flake.yml`
- flakestry.dev is also supported by `publish-flake.yml`

.github/workflows/flakehub-backfill.yaml

@@ -1,25 +0,0 @@
-name: "Push to FlakeHub (manual)"
-
-on:
-  workflow_dispatch:
-    inputs:
-      tag:
-        description: "Tag to publish to FlakeHub"
-        type: "string"
-        required: true
-
-jobs:
-  push:
-    runs-on: "ubuntu-22.04"
-    permissions:
-      id-token: "write"
-      contents: "read"
-    steps:
-      - uses: "actions/checkout@v4"
-        with:
-          ref: "refs/tags/${{ inputs.tag }}"
-      - uses: "DeterminateSystems/nix-installer-action@main"
-      - uses: "DeterminateSystems/flakehub-push@main"
-        with:
-          visibility: "public"
-          tag: "${{ inputs.tag }}"

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

@@ -1,17 +0,0 @@
-on: 
-  push:
-    tags:
-      - "v*.*.*"
-
-jobs:
-  publish:
-    runs-on: "ubuntu-latest"
-    permissions:
-      id-token: "write"
-      contents: "read"
-    steps:
-      - uses: "actions/checkout@v4"
-      - uses: "DeterminateSystems/nix-installer-action@main"
-      - uses: "DeterminateSystems/flakehub-push@main"
-        with:
-          visibility: "public"

.github/workflows/publish-flake.yml

@@ -0,0 +1,36 @@
+name: "Publish flake"
+on:
+  push:
+    tags:
+      - "v?[0-9]+.[0-9]+.[0-9]+"
+      - "v?[0-9]+.[0-9]+.[0-9]+*"
+  workflow_dispatch:
+    inputs:
+      tag:
+        description: "Existing tag to publish"
+        type: "string"
+        required: true
+jobs:
+  publish-flakestry:
+    name: "Publish to `flakestry.dev`"
+    runs-on: ubuntu-latest
+    permissions:
+      id-token: "write"
+      contents: "read"
+    steps:
+      - uses: flakestry/flakestry-publish@main
+        with:
+          version: "${{ inputs.tag || github.ref_name }}"
+  publish-flakehub:
+    name: "Publish to `flakehub.com`"
+    runs-on: ubuntu-latest
+    permissions:
+      id-token: "write"
+      contents: "read"
+    steps:
+      - uses: "actions/checkout@v4"
+      - uses: "DeterminateSystems/nix-installer-action@main"
+      - uses: "DeterminateSystems/flakehub-push@main"
+        with:
+          visibility: "public"
+          tag: "${{ inputs.tag || github.ref_name }}"