macOS CI: fails to correctly link bazelisk

Azure pipelines agents are not correctly installing/linking bazelisk via https://github.com/envoyproxy/envoy/blob/master/ci/mac_ci_setup.sh. All calls to `bazel` are using the installed system binary, not the version requested by `.bazelversion`. This is not an issue in master currently because what is installed and the `.bazelversion` is the same (3.1.0). However, if the versions are different problems arise, e.g: https://github.com/envoyproxy/envoy/pull/11208