Don't assume that the RUBYLIB environment variable exists

When run under asdf-direnv with direnv's `strict_env` set to true, then
this script will run with bash's `nounset` enabled, which will cause
this script to crash in the following way:

  ./exec-env:6: RUBYLIB: unbound variable

(see asdf-community/asdf-direnv#115 for
details)

I know this doesn't affect regular asdf users, but this seems like a
reasonable change to make, I hope you'll accept it!

bin/exec-env

@@ -3,6 +3,7 @@
 current_script_path=${BASH_SOURCE[0]}
 ruby_plugin_dir=$(dirname "$(dirname "$current_script_path")")
 
+RUBYLIB=${RUBYLIB:-}
 if [ "$RUBYLIB" = "" ]; then
   export RUBYLIB="$ruby_plugin_dir/rubygems-plugin"
 else