Add workflow to re-run Buildmaster builds

.github/workflows/buildmaster-rebuild.yml

@@ -0,0 +1,35 @@
+name: Buildmaster rebuild
+
+on:
+  workflow_dispatch:
+    inputs:
+      pull_request:
+        description: Pull request ID
+        type: number
+        required: true  # remove if we support commit rebuilds later
+
+jobs:
+  rebuild:
+    runs-on: ubuntu-latest
+    name: Trigger Buildmaster
+    steps:
+    - name: Set up Python 3
+      uses: actions/setup-python@v2
+      with:
+        python-version: 3
+    - name: Install dependencies
+      run: |
+        pip install requests
+    - name: Clone DFHack
+      uses: actions/checkout@v1
+      with:
+        fetch-depth: 1
+        submodules: false
+    - name: Run
+      env:
+        DFHACK_BUILDMASTER_WEBHOOK_URL: ${{ secrets.DFHACK_BUILDMASTER_WEBHOOK_URL }}
+        DFHACK_BUILDMASTER_WEBHOOK_SECRET: ${{ secrets.DFHACK_BUILDMASTER_WEBHOOK_SECRET }}
+        GITHUB_REPO: ${{ github.repository }}
+        GITHUB_TOKEN: ${{ github.token }}
+      run: |
+        python ci/buildmaster-rebuild-pr.py --pull-request ${{ inputs.pull_request }}