bump up rootlesskit to v0.9.4

Now `rootlesskit-docker-proxy` returns detailed error message on
exposing privileged ports: https://github.com/rootless-containers/rootlesskit/pull/136

Full changes: https://github.com/rootless-containers/rootlesskit/compare/v0.9.2...v0.9.4

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

hack/dockerfile/install/rootlesskit.installer

@@ -1,7 +1,7 @@
 #!/bin/sh
 
-# v0.9.2
-: ${ROOTLESSKIT_COMMIT:=eefbc3f7fb73d9a993605c9ff9a36bfcad6c1270}
+# v0.9.4
+: ${ROOTLESSKIT_COMMIT:=0fec9adac6b078aa8616da47e9d75f663ca3f492}
 
 install_rootlesskit() {
 	case "$1" in