allow doc flags to be set by env var
allow doc flags to be set by env var

This will allow things like --hide-source from issue #19019
to be set from packaging scripts, etc.

andrsd/moose,milljm/moose,laagesen/moose,dschwen/moose,harterj/moose,laagesen/moose,idaholab/moose,andrsd/moose,sapitts/moose,milljm/moose,jessecarterMOOSE/moose,bwspenc/moose,bwspenc/moose,dschwen/moose,harterj/moose,lindsayad/moose,dschwen/moose,lindsayad/moose,nuclear-wizard/moose,sapitts/moose,jessecarterMOOSE/moose,laagesen/moose,jessecarterMOOSE/moose,nuclear-wizard/moose,harterj/moose,harterj/moose,jessecarterMOOSE/moose,sapitts/moose,andrsd/moose,milljm/moose,andrsd/moose,bwspenc/moose,bwspenc/moose,idaholab/moose,idaholab/moose,bwspenc/moose,milljm/moose,andrsd/moose,lindsayad/moose,harterj/moose,laagesen/moose,idaholab/moose,laagesen/moose,sapitts/moose,sapitts/moose,idaholab/moose,nuclear-wizard/moose,nuclear-wizard/moose,milljm/moose,lindsayad/moose,dschwen/moose,dschwen/moose,lindsayad/moose,jessecarterMOOSE/moose