Add appcast.xml generation action (#202)

This adds an action to generate the appcast.xml and push it to cloud
storage bucket. This replaces the thing currently running on
build.syncthing.net, which is in a slow death spiral.

I set the action to run nightly and manually; since it's based on the
published releases it should be fine to run periodically (generating
identical contents each time) and manually in case a quick update is
required.

(Also refreshed the dependencies for the generation script while I was
in there.)

.github/workflows/generate-appcast.yml

@@ -0,0 +1,31 @@
+name: Generate appcast.xml
+
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: '42 3 * * *'
+
+jobs:
+  generate-appcast:
+    name: Generate appcast.xml
+    runs-on: ubuntu-latest
+    environment: appcast
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Generate appcast.xml
+        run: |
+          cd cmd/ghreleases2appcast
+          go run . -o ../../appcast.xml
+
+      - name: Push to Spaces
+        uses: docker://docker.io/rclone/rclone:latest
+        env:
+          RCLONE_CONFIG_SPACES_TYPE: s3
+          RCLONE_CONFIG_SPACES_PROVIDER: DigitalOcean
+          RCLONE_CONFIG_SPACES_ACCESS_KEY_ID: ${{ secrets.SPACES_KEY }}
+          RCLONE_CONFIG_SPACES_SECRET_ACCESS_KEY: ${{ secrets.SPACES_SECRET }}
+          RCLONE_CONFIG_SPACES_ENDPOINT: ams3.digitaloceanspaces.com
+          RCLONE_CONFIG_SPACES_ACL: public-read
+        with:
+          args: copy appcast.xml spaces:syncthing-macos/appcast/

cmd/ghreleases2appcast/go.mod

@@ -1,11 +1,17 @@
 module github.com/syncthing/syncthing-macos/cmd/ghreleases2appcast
 
-go 1.15
+go 1.21
 
 require (
 	github.com/google/go-github v17.0.0+incompatible
-	github.com/google/go-querystring v1.0.0 // indirect
-	github.com/hashicorp/go-version v1.2.1
+	github.com/hashicorp/go-version v1.6.0
 	github.com/russross/blackfriday/v2 v2.1.0
-	golang.org/x/oauth2 v0.0.0-20210402161424-2e8d93401602
+	golang.org/x/oauth2 v0.12.0
+)
+
+require (
+	github.com/golang/protobuf v1.5.3 // indirect
+	github.com/google/go-querystring v1.1.0 // indirect
+	google.golang.org/appengine v1.6.8 // indirect
+	google.golang.org/protobuf v1.31.0 // indirect
 )