Port CI to Github Actions (#3363)

This is intended to replace the existing Buildkite one, but the latter can be removed at a later point when all the bits are in place.

.github/workflows/main.yml

@@ -0,0 +1,67 @@
+name: "Spec"
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  build-openapi:
+    name: "🐍 Build OpenAPI definitions for matrix.org"
+    runs-on: ubuntu-latest
+    container: "python:3.9"
+    steps:
+      - name: "📥 Source checkout"
+        uses: actions/checkout@v2
+      - name: "📦 Asset creation"
+        run: |
+          python3 -m venv env && . env/bin/activate
+          pip install -r scripts/requirements.txt
+          scripts/generate-matrix-org-assets
+      - name: "📤 Artifact upload"
+        uses: actions/upload-artifact@v2
+        with:
+          name: openapi-artifact
+          path: assets.tar.gz
+
+  build-spec:
+    name: "📖 Build the spec"
+    runs-on: ubuntu-latest
+    steps:
+      - name: "➕ Setup Node"
+        uses: actions/setup-node@v2
+        with:
+          node-version: '14'
+      - name: "➕ Setup Hugo"
+        uses: peaceiris/actions-hugo@c03b5dbed22245418539b65eb9a3b1d5fdd9a0a6
+        with:
+          hugo-version: '0.85.0'
+          extended: true
+      - name: "📥 Source checkout"
+        uses: actions/checkout@v2
+        with:
+          submodules: 'recursive'
+      - name: "⚙️ npm"
+        run: |
+          npm i
+          npm run get-proposals
+      - name: "⚙️ hugo"
+        run: hugo --baseURL "/unstable" -d "spec"
+      - name: "📦 Tarball creation"
+        run: tar -czf spec.tar.gz spec
+      - name: "📤 Artifact upload"
+        uses: actions/upload-artifact@v2
+        with:
+          name: spec-artifact
+          path: spec.tar.gz
+
+  rebuild-matrixdotorg:
+    name: "👷 Rebuild matrix.org"
+    needs: build-openapi
+    runs-on: ubuntu-latest
+    if: ${{ false && github.event_name == 'push' && github.ref == 'refs/head/main' }}
+    steps:
+      - name: "🪄 Triggering rebuild of matrix.org"
+        run: |
+          curl -XPOST -u "${{secrets.TRIGGER_MATRIXORG_REBUILD_TOKEN}}" -H "Accept: application/vnd.github.everest-preview+json" -H "Content-Type: application/json" https://api.github.com/repos/${{ github.repository_owner }}/matrix.org/actions/workflows/build-matrix.org.yml/dispatches --data '{"ref": "${{ github.ref }}" }'