use cs50/cli:canary-amd64 base image for branch testing

Author: rongxin-liu

.github/workflows/main.yml

@@ -68,7 +68,7 @@ jobs:
       - name: Build (${{ github.ref_name}})
         if: ${{ github.ref != 'refs/heads/main' && github.ref != 'refs/heads/canary' }}
         run: |
-          export TAG=amd64
+          export TAG=canary-amd64
           export VCS_REF=$(git rev-parse HEAD)
           npm install -g @devcontainers/cli
           devcontainer build --workspace-folder . --config devcontainer.json --image-name cs50/codespace:${{ github.sha }} --image-name cs50/codespace:${{ github.ref_name }}