ci: use absolute paths for river workflow

Author: sanity

.github/workflows/six-peer-regression.yml

@@ -36,7 +36,7 @@ jobs:
           override: true
 
       - name: Run six-peer regression
-        working-directory: river/main
+        working-directory: ${{ github.workspace }}/river/main
         env:
           FREENET_CORE_PATH: ${{ github.workspace }}/freenet-core
           RUST_LOG: info