ci: add reuse job

As mentioned in the previous commit, `reuse lint` can be used to ensure
that copyright information is always present and up to date.

This adds a GitHub Action that does just that, using the official
fsfe/reuse-action

.github/workflows/ci.yml

@@ -24,3 +24,9 @@ jobs:
         run: ./.ci/scripts/transifex/docker.sh
         env:
           TRANSIFEX_API_TOKEN: ${{ secrets.TRANSIFEX_API_TOKEN }}
+
+  reuse:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - uses: fsfe/reuse-action@v1