3018 Pro Z-axis spindle carriage 38mm on center bearings ~12mm heat insert

I fixed the Z-axis for the remix model to fit my 3018 Pro (from TwoTrees, YMMV check the measurements before printing):

• Made a space for the Z-axis anti-backlash lock/T-nut
• Adjusted the size of the heat insert nut bore to accomadate the size I have (~12mm outer diameter)
• Adjusted the linear bearings to be 38mm on center apart (the original model I remixed was 40mm on center)