NIX_REMOTE_SYSTEMS: actually support multiple :-separated entries

Bug not reported in 6 years, but here you go.

Also it is safe to switch to normal concatStringsSep behavior
because tokenizeString does not produce empty items.

src/libstore/globals.cc

@@ -35,6 +35,8 @@
 #include <sys/sysctl.h>
 #endif
 
+#include "strings.hh"
+
 namespace nix {
 
 
@@ -82,7 +84,7 @@ Settings::Settings()
         Strings ss;
         for (auto & p : tokenizeString<Strings>(*s, ":"))
             ss.push_back("@" + p);
-        builders = dropEmptyInitThenConcatStringsSep(" ", ss);
+        builders = concatStringsSep("\n", ss);
     }
 
 #if defined(__linux__) && defined(SANDBOX_SHELL)