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.

validator_client/slashing_protection/Cargo.toml

@@ -27,3 +27,4 @@ rayon = { workspace = true }
 
 [features]
 arbitrary-fuzz = ["types/arbitrary-fuzz"]
+portable = ["types/portable"]