Replace $NIX_REMOTE_SYSTEMS with an option "builder-files"

Also, to unify with hydra-queue-runner, allow it to be a list of
files.

src/libstore/globals.hh

@@ -133,6 +133,10 @@ public:
     Setting<std::string> builders{this, "", "builders",
         "A semicolon-separated list of build machines, in the format of nix.machines."};
 
+    Setting<Strings> builderFiles{this,
+        {nixConfDir + "/machines"}, "builder-files",
+        "A list of files specifying build machines."};
+
     Setting<off_t> reservedSize{this, 8 * 1024 * 1024, "gc-reserved-space",
         "Amount of reserved disk space for the garbage collector."};