Suggest --incompatible_strict_action_env in Bazel readme

The flag implies a slightly more consistent env for bazel actions,
which while not supported by all bazel libs, can increase cache hit
percentage in those that do. rabbitmq-server builds fine with the flag
enabled, as we don't depend on any such incompatible libraries

BAZEL.md

@@ -30,6 +30,7 @@ build --@bazel-erlang//:erlang_home=/path/to/erlang/installation
 build --@bazel-erlang//:erlang_version=23.1
 build --@bazel-erlang//:elixir_home=/path/to/elixir/installation
 build --test_strategy=exclusive
+build --incompatible_strict_action_env
 ```
 
 Additionally, on **macOS**, you will likely need to add