Add env-filter feature flag to tracing-subscriber

[macladson]

Issue Addressed

unstable does not currently compile:

error[E0433]: failed to resolve: could not find `EnvFilter` in `tracing_subscriber`
   --> lighthouse/src/main.rs:369:50
    |
369 |     let filter_layer = match tracing_subscriber::EnvFilter::try_from_default_env()
    |                                                  ^^^^^^^^^ could not find `EnvFilter` in `tracing_subscriber`

I believe the issue is that tracing_subscriber::EnvFilter relies on the env-filter feature which wasn't added in #4979

Proposed Changes

Add the env-filter feature flag to the tracing-subscriber crate.

[macladson]

Closing since this will be fixed once #4988 merges.