ci: Dependabot pull requests regenerate THIRD-PARTY-LICENSES.txt; num-bigint moves with num-rational

Every Cargo update changes the licences file, which Dependabot cannot regenerate, so all its pull
requests failed the licences check before testing anything. CI regenerates it for them and a new
workflow commits it. num-bigint 0.5 alone cannot build: the predicates' BigInt is num-rational's.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

.github/workflows/ci.yml

@@ -60,6 +60,12 @@ jobs:
       - uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
         with:
           save-if: ${{ github.ref == 'refs/heads/develop' }}
+      # A dependency update changes THIRD-PARTY-LICENSES.txt, which Dependabot cannot regenerate:
+      # regenerate it here so the gate checks everything else, and dependabot-licences.yml commits
+      # it to the pull request.
+      - name: Regenerate THIRD-PARTY-LICENSES.txt (Dependabot)
+        if: github.event.pull_request.user.login == 'dependabot[bot]'
+        run: cargo xtask licences
       - run: cargo xtask ci
 
   heavy: