FindRuby: Restore compatibility variable RUBY_VENDORLIB_DIR
Since commit e672db628b (FindRuby: Rename variables to match case of
module name, 2020-03-11, v3.18.0-rc1~546^2), the result variables named
with the old `RUBY_` prefix are provided by compatibility code that maps
from the new `Ruby_` prefix variables. This variable was accidentally
left out.
Fixes: #21369