Avoid the following warning on every startup:

Warning: AppCDS archive directory is not writable, disabling AppCDS operations

Users can set the JRUBY_JSA environment variable to a writeable file; doing so
improves performance significantly.

Index: bin/jruby.sh
--- bin/jruby.sh.orig
+++ bin/jruby.sh
@@ -875,6 +875,7 @@ if [ -n "${JRUBY_JSA-}" ]; then
 else
     # Default JVM Class Data Sharing Archive (jsa) file for JVMs that support it
     jruby_jsa_file="$JRUBY_HOME/lib/jruby-java$java_runtime_version.jsa"
+    use_jsa_file=false
 fi
 readonly jruby_jsa_file
 
