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).

tests/build-remote.sh

@@ -53,3 +53,16 @@ nix path-info --store $TEST_ROOT/machine3 --all \
   | grep -v builder-build-remote-input-1.sh \
   | grep -v builder-build-remote-input-2.sh \
   | grep builder-build-remote-input-3.sh
+
+# Behavior of keep-failed
+out="$(nix-build 2>&1 failing.nix \
+  --builders "$(join_by '; ' "${builders[@]}")"  \
+  --keep-failed \
+  --store $TEST_ROOT/machine0 \
+  -j0 \
+  --arg busybox $busybox)" || true
+
+[[ "$out" =~ .*"note: keeping build directory".* ]]
+
+build_dir="$(grep "note: keeping build" <<< "$out" | sed -E "s/^(.*)note: keeping build directory '(.*)'(.*)$/\2/")"
+[[ "foo" = $(<"$build_dir"/bar) ]]