Add back "Configure download mirrors" step to test.tml

Accidentally dropped in a pre-merge rebase of #1283.

.github/workflows/test.yml

@@ -90,6 +90,13 @@ jobs:
         # Set a custom output root directory to avoid long file name issues.
         run: |
           [System.IO.File]::WriteAllLines((Join-Path -Path $env:USERPROFILE -ChildPath '.bazelrc'), 'startup --output_user_root=C:/tmp')
+      - name: Configure download mirrors
+        shell: bash
+        run: |
+          if [ ! -z "${{ secrets.WORKERS_MIRROR_URL }}" ] ; then
+            # Strip comment in front of WORKERS_MIRROR_URL, then substitute secret to use it.
+            sed -e '/WORKERS_MIRROR_URL/ { s@# *@@; s@WORKERS_MIRROR_URL@${{ secrets.WORKERS_MIRROR_URL }}@; }' -i.bak WORKSPACE
+          fi
       - name: Generate list of excluded Bazel targets
         # Exclude large benchmarking binaries created in debug and asan configurations to avoid
         # running out of disk space on the runner (nominally 14GB). We typically have two copies