Enable socket-only daemon operations without filesystem access

Allows the Nix daemon to serve store paths purely over Unix domain
sockets without requiring the client to have filesystem access to
the store directory. This can be useful for VM setups where the host
serves paths to the guest via socket.

Tests verify socket-only operations work for copying, substitution,
and remote building (tested on Linux), with both local and binary cache stores.

tests/functional/socket-only-daemon-binary-cache.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+
+# Run the socket-only-daemon test with a binary cache backend
+# This tests that
+# - The daemon can serve a binary cache store; not just a local store
+# - Client's store operations do not reach into the file system secretly,
+#   because the files don't exist in the places where a local store would
+#   put them. (We have NARs instead)
+
+daemon_backing_store_is_binary_cache=1
+source socket-only-daemon.sh