workflows/tests: set HOMEBREW_DOWNLOAD_CONCURRENCY for test-bot tests.

This should give us some basic integration testing for the new download
concurrency code before we enabled/test it in homebrew-core's CI.

Author: MikeMcQuaid

.github/workflows/tests.yml

@@ -368,6 +368,7 @@ jobs:
     env:
       HOMEBREW_TEST_BOT_ANALYTICS: 1
       HOMEBREW_ENFORCE_SBOM: 1
+      HOMEBREW_DOWNLOAD_CONCURRENCY: 4
     steps:
       - name: Install Homebrew and Homebrew's dependencies
         # All other images are built from our Homebrew Dockerfile.