[CI] Install deps for backward compatibility check

On re-enabling the backward compatibility check that came with the
ComfyUI cookiecutter project, it failed due to missing dependencies
(specifically pytorch).

So install dependencies before running the action. Choose dependencies
from the PR rather than main, in case they are different, in the spirit
of testing backward compatibility.

Signed-off-by: David Feltell <david.feltell@foundry.com>

Author: feltech

.github/workflows/validate.yml

@@ -14,4 +14,34 @@ jobs:
   validate:
     runs-on: ubuntu-latest
     steps:
+      # We cannot simply run the node-diff action, since it will try
+      # to import our package without installing dependencies. So we
+      # install dependencies ourselves first.
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.10"  # Same as comfy-org/node-diff
+
+      - name: Checkout PR
+        uses: actions/checkout@v4
+        with:
+          path: pr_repo  # Same as comfy-org/node-diff
+
+      - uses: actions/checkout@v4
+        name: Checkout ComfyUI
+        with:
+          repository: comfyanonymous/ComfyUI
+          ref: v0.3.57
+          path: comfyui
+
+      - name: Install dependencies
+        shell: bash
+        run: |
+          python -m pip install --upgrade pip
+          pip install ./pr_repo
+          pip install -r ./comfyui/requirements.txt
+
       - uses: comfy-org/node-diff@main
+        env:
+          PYTHONPATH: ${{ github.workspace }}/comfyui