Temporarily allow CI failures for iOS (#142365)

iOS tests are flaky right now. Based on the convo in Discord, it seems like allowing failures is the best option.

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

Author: savannahostrowski

.github/workflows/build.yml

@@ -706,6 +706,7 @@ jobs:
       uses: re-actors/alls-green@05ac9388f0aebcb5727afa17fcccfecd6f8ec5fe
       with:
         allowed-failures: >-
+          build-ios,
           build-windows-msi,
           build-ubuntu-ssltests-awslc,
           build-ubuntu-ssltests-openssl,