ci: build Debian packages for pull request

Add workflow that will build Debian packages for each pull request and upload
them as artifacts to the pull request.

Part of timescale#16

.github/workflows/debuild.yml

@@ -0,0 +1,18 @@
+name: Debuild Release
+on:
+  pull_request:
+    branches: [main, master]
+jobs:
+  debuild:
+    runs-on: ubuntu-latest
+    environment: main
+    steps:
+      - name: Install dependencies
+        run: sudo apt-get install -qy equivs devscripts debmake postgresql-all postgresql-server-dev-all
+      - uses: actions/checkout@v2
+      - name: Build unsigned package
+        run: debmake -t -i "debuild -i -uc -us"
+      - uses: actions/upload-artifact@v3
+        with:
+          name: debian
+          path: ../postgresql-*pg-influx*.{ddeb,deb,dsc,buildinfo,changes}