daemon: Add 'built-in-builders' RPC.
* nix/libstore/builtins.cc (builtinBuilderNames): New function.
* nix/libstore/builtins.hh (builtinBuilderNames): New declaration.
* nix/libstore/worker-protocol.hh (PROTOCOL_VERSION): Bump to 0x160.
(WorkerOp)[wopBuiltinBuilders]: New value.
* nix/nix-daemon/nix-daemon.cc (performOp): Handle it.
* guix/store.scm (operation-id)[built-in-builders]: New value.
* guix/store.scm (read-arg): Add 'string-list'.
(built-in-builders): New procedure.
* tests/derivations.scm ("built-in-builders"): New test.
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