charon and lighthouse_vc fixes

[b0a7]

This PR fixes a couple of errors encountered when trying to use the geth_teku_lighthouse example.

1 - lighthouse validator was failing to start with error: "lighthouse_vc-1 | error: unexpected value 'true' for '--builder-proposals' found; no more were expected" Fix by modifying lighthouse/run.sh to check the value of the variable and add the --builder-proposals accordingly (without =true)

2 - charon was not receiving the BUILDER_API_ENABLED env variable properly. Fix by using the proper variable name (as per Lodestar example)

lighthouse validator also has a 'distributed' arg, which I also added because it looks to be required for use in a DV

[b0a7]

note that I am just using charon and lighthouse-vc, so there may be additional updates required for the geth, teku & mev services

[KaloyanTanev]

Hey @b0a7 , we have started work for multi client support, check #344 . We are aiming for much more generic way of swapping clients throughout the stack.