Fix checkpatch in CI

.github/workflows/checkpatch.yml

@@ -5,13 +5,20 @@ jobs:
   checkpatch:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - name: Set up repo
+        run: |
+          git clone -b "${{ github.event.pull_request.head.ref }}" "${{ github.event.pull_request.head.repo.clone_url }}" rgbds
+          cd rgbds
+          git remote add upstream "${{ github.event.pull_request.base.repo.clone_url }}"
+          git fetch upstream
       - name: Set up checkpatch
+        working-directory: rgbds
         run: |
           wget 'https://raw.githubusercontent.com/torvalds/linux/master/scripts/checkpatch.pl'
           chmod +x checkpatch.pl
-          touch const_structs.checkpatch
+          wget 'https://raw.githubusercontent.com/torvalds/linux/master/scripts/const_structs.checkpatch'
-          touch spelling.txt
+          wget 'https://raw.githubusercontent.com/torvalds/linux/master/scripts/spelling.txt'
       - name: Checkpatch
+        working-directory: rgbds
         run: |
-          make checkpatch CHECKPATCH=./checkpatch.pl BASE_REF=${{ github.base_ref }} Q=
+          make checkpatch CHECKPATCH=./checkpatch.pl "BASE_REF=${{ github.event.pull_request.base.sha }}" Q=