BUild the vacuum window differently to avoid G4 runtime warnings.

Gean does not like booleans with surfaces that are common to both components.
An example warning:

-------- WWWW ------- G4Exception-START -------- WWWW -------
*** G4Exception : GeomSolids1002
      issued by : G4Paraboloid::DistanceToIn(p,v)
Point p is inside! - GabsBulgeGas
          p = (-5.81848,-1.9882,-4.7605) mm
          v = (-0.0515649,-0.0085554,0.998633) mm
*** This is just a warning message. ***
-------- WWWW -------- G4Exception-END --------- WWWW -------