Merge branch 'mariadbPlugins' into 'master'

setup.[c]sh: add MARIADB_PLUGIN_DIR for the mariadb / mysql client to find plugin libraries

See merge request !2852

(cherry picked from commit 7f1e357a)

efb641f6 setup.[c]sh: add MARIADB_PLUGIN_DIR for the mariadb / mysql client to find plugin libraries

Co-authored-by: Andre Sailer <andre.philippe.sailer@cern.ch>