ci: make cache paths relative to workspace

workspace path can be different when running locally, thus preventing successful caching

.forgejo/workflows-composite/build-backend/action.yaml

@@ -6,7 +6,7 @@ runs:
     - uses: actions/cache@v4
       id: cache-backend
       with:
-        path: '/workspace/forgejo/forgejo/gitea'
+        path: ${{github.workspace}}/gitea
         key: backend-build-${{ github.sha }}
     - if: steps.cache-backend.outputs.cache-hit != 'true'
       run: |

.forgejo/workflows/testing.yml

@@ -39,7 +39,7 @@ jobs:
       - name: "Cache frontend build for playwright testing"
         uses: actions/cache/save@v4
         with:
-          path: '/workspace/forgejo/forgejo/public/assets'
+          path: ${{github.workspace}}/public/assets
           key: frontend-build-${{ github.sha }}
   test-unit:
     if: ${{ !startsWith(vars.ROLE, 'forgejo-') }}
@@ -95,7 +95,7 @@ jobs:
         uses: actions/cache/restore@v4
         id: cache-frontend
         with:
-          path: '/workspace/forgejo/forgejo/public/assets'
+          path: ${{github.workspace}}/public/assets
           key: frontend-build-${{ github.sha }}
       - name: "Build frontend (if not cached)"
         if: steps.cache-frontend.outputs.cache-hit != 'true'