nix-store --serve: pass on settings.keepFailed from SSH store

When doing e.g.

    nix-build -A package --keep-failed --option \
      builders \
      'ssh://mfhydra?remote-store=/home/bosch/store x86_64-linux - 10 4 big-parallel'

this doesn't work properly because this build-setting is ignored.

I changed this behavior by passing the `settings.keepFailed` through the
serve-protocol to remote machines to make sure that I can introspect the
build-directory (which is particularly helpful when I have to look at a
`config.log` from a failed build for instance).

src/nix-store/nix-store.cc

@@ -801,6 +801,9 @@ static void opServe(Strings opFlags, Strings opArgs)
             settings.enforceDeterminism = readInt(in);
             settings.runDiffHook = true;
         }
+        if (GET_PROTOCOL_MINOR(clientVersion) >= 7) {
+            settings.keepFailed = (bool) readInt(in);
+        }
         settings.printRepeatedBuilds = false;
     };