worfklows: add flakehub integration. (gmodena#50)

Add an integration workflow with flakehub.
Change nix installer Github Action to
DeterminateSystems.

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

@@ -0,0 +1,28 @@
+name: "Publish tags to FlakeHub"
+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:
+  flakehub-publish:
+    runs-on: "ubuntu-latest"
+    permissions:
+      id-token: "write"
+      contents: "read"
+    steps:
+      - uses: "actions/checkout@v3"
+        with:
+          ref: "${{ (inputs.tag != null) && format('refs/tags/{0}', inputs.tag) || '' }}"
+      - uses: "DeterminateSystems/nix-installer-action@main"
+      - uses: "DeterminateSystems/flakehub-push@main"
+        with:
+          visibility: "public"
+          name: "gmodena/nix-flatpak"
+          tag: "${{ inputs.tag }}"
+

.github/workflows/test.yml

@@ -18,7 +18,7 @@ jobs:
     continue-on-error: true
     steps:
     - uses: actions/checkout@v3
-    - uses: cachix/install-nix-action@v18
+    - uses: DeterminateSystems/nix-installer-action@v4
       with:
         github_access_token: ${{ secrets.GITHUB_TOKEN }}
     - run: nix flake check