Fix modal SASS when $spacer is of the pixels unit

Created by: TomONeill

Setting the $spacer variable to a pixels unit breaks the compilation due to a calculation that's happening in the modal SASS:

SassError: 8px and 0.25rem have incompatible units.
    ╷
131 │   padding: $modal-inner-padding - $modal-footer-margin-between * .5;
    │            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ╵
  node_modules\bootstrap\scss\_modal.scss 131:12  @import
  .scss\bootstrap.scss 29:9                       @import

It's actually line 132 now (click).

Since it's half the size of the spacer, might as well calculate the size.