fix(ci): correct backend build directory in E2E workflow

The E2E workflow was failing during backend build because make build
was being executed from the backend/ directory, but the Makefile exists
at the root level.

Remove working-directory: backend from Build backend step
Allows make build to execute from root where Makefile is located
Verified with local test: frontend + backend build successfully
Related to PR #550 E2E workflow failures

.github/workflows/e2e-tests.yml

@@ -108,7 +108,6 @@ jobs:
 
       - name: Build backend
         run: make build
-        working-directory: backend
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f # v3