workflows/tests: set HOMEBREW_ARM64_TESTING

We now have arm64 Linux CI in PRs for formulae with `:arm64_linux`
bottles. We need to set this to make sure they don't fail `brew doctor`.

See, for example, #213532.

Author: carlocab

.github/workflows/tests.yml

@@ -15,6 +15,7 @@ env:
   HOMEBREW_TEST_BOT_ANALYTICS: 1
   HOMEBREW_ENFORCE_SBOM: 1
   HOMEBREW_NO_BUILD_ERROR_ISSUES: 1
+  HOMEBREW_ARM64_TESTING: 1
   GH_REPO: ${{github.repository}}
   GH_NO_UPDATE_NOTIFIER: 1
   GH_PROMPT_DISABLED: 1