Handle BLS portable vs modern distinction in CI cache

[michaelsproul]

Description

We've started using build caches for the local-testnet job, which seems to be a good testing ground before we apply caches more broadly.

Currently there's an issue where the cache can be transferred from a machine with ADX to a machine without, causing a SIGILL (exit code 132): https://github.com/sigp/lighthouse/runs/5545911178?check_suite_focus=true

I think we should update the cache key so that it includes the machine's ADX status. That way there won't be any cache sharing between machines with ADX and without. Although many hypervisors lie about ADX compatibility in their CPUID it's sufficient for us to rely on the CPUID as that's what the BLST library uses to enable/disable ADX (see: https://github.com/supranational/blst/blob/f2380d22a1c6a5f82c1bc187ec7fd4019051969e/bindings/rust/build.rs#L102-L109).