Automatic merge from submit-queue (batch tested with PRs 45684, 45266, 45669, 44787, 44984)

Fix XDG-based kubectl plugin dirs

XDGDataPluginLoader messed up its default-value handling for `XDG_DATA_DIRS` and ends up scanning *all of /usr/share* looking for plugins if you don't have that set :-O

/release-note-none
/assign @fabianofranz