Enable BLS portable feature on all CI tests (#4868)

## Issue Addressed

Addresses the recent CI failures caused by caching `blst` for the wrong CPU type. 

## Proposed Changes

- Use `FEATURES: jemalloc,portable` when building Lighthouse & `lcli` in tests
- Add a new `TEST_FEATURES` and set to `portable` for all CI test jobs.
- Updated Makefiles to read the `TEST_FEATURES` environment variable, and default to none.

beacon_node/beacon_chain/Cargo.toml

@@ -10,6 +10,7 @@ default = ["participation_metrics"]
 write_ssz_files = []  # Writes debugging .ssz files to /tmp during block processing.
 participation_metrics = []  # Exposes validator participation metrics to Prometheus.
 fork_from_env = [] # Initialise the harness chain spec from the FORK_NAME env variable
+portable = ["bls/supranational-portable"]
 
 [dev-dependencies]
 maplit = { workspace = true }